Valve assembly for controlling actuation of an actuator of a track tensioning system

ABSTRACT

A valve assembly for controlling actuation of an actuator of a track tensioning system for adjusting tension on a drive track chain includes a valve housing defining (i) a master chamber, (ii) a slave chamber, (iii) a fluid passage which fluidly couples the master chamber to the slave chamber, and (iv) a fluid outlet which fluidly couples the slave chamber to the actuator. The valve assembly also includes a master valve member positioned within the master chamber. The master valve member (i) isolates the master chamber from the slave chamber when the master valve member is positioned in a first master position, and (ii) places the master chamber in fluid communication with the slave chamber when the master valve member is positioned in a second master position. The valve assembly further includes a slave piston positioned within the slave chamber, the slave piston (i) isolates the fluid outlet from the slave chamber when the slave piston is positioned in a first slave position, and (ii) places the fluid outlet in fluid communication with the slave chamber when the slave piston is positioned in a second slave position. Movement of the master valve member from the first master position to the second master position causes movement of the slave piston from the first slave position to the second slave position. Movement of the slave piston from the first slave position to the second slave position causes actuation of the actuator, whereby tension on the drive track chain is adjusted by the actuator.

CROSS REFERENCE

Cross reference is made to copending U.S. patent applications Ser. No.09/464,964, entitled “Apparatus and Method for Adjusting Tension of aDrive Track Chain of a Work Machine Which Utilizes a Sensor for SensingPosition of an Undercarriage Component” by Thomas E. Oertley; Ser. No.09/464,965, entitled “Apparatus and Method for Operating a HydraulicExcavator Which Has a Position Sensor for Sensing Position of an IdlerWheel” by Thomas E. Oertley; Ser. No. 09/465,467, entitled “Apparatusand Method for Operating a Track Tensioning Assembly of a HydraulicExcavator” by Thomas E. Oertley; Ser. No. 09/464,966, entitled “TrackTensioning Assembly for Adjusting Tension on a Drive Track Chain of aWork Machine Having an Actuator Which Includes a Pair of ConcentricallyArranged Pistons” by Thomas E. Oertley; and Ser. No. 09/464,967,entitled “Track Tensioning Assembly for Adjusting Tension on a DriveTrack Chain of a Work Machine Having a Slack Adjuster Device AssociatedTherewith” by Clifford E. Miller, each of which is assigned to the sameassignee as the present invention, and each of which is filedconcurrently herewith.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to a drive track chain of a workmachine, and more particularly to a valve assembly for controllingactuation of an actuator of a track tensioning system.

BACKGROUND OF THE INVENTION

A work machine, such as a track-type tractor or excavator, is typicallysupported and propelled by a pair of undercarriage assemblies, each oneof which includes an endless drive track chain having a plurality ofinterconnected articulating components or links. The undercarriageassembly typically also includes a drive sprocket and one or more idlerwheels, around each of which the drive track chain is advanced.

During operation of the work machine, it is necessary to maintaintension on the drive track chain in order to keep the chain fromderailing from the drive sprocket and/or the idler rollers. In order tomaintain tension on the drive track chain, a tension adjustmentmechanism such as a hydraulic cylinder or coiled spring is oftenincluded in heretofore designed undercarriages. In particular, thecylinder or coiled spring urges the front idler roller in a directionaway from the rear idler roller (or rear drive sprocket in the case ofan excavator) thereby creating tension on the track chain.

Over a period of time, a number of the components associated with theundercarriage assembly, such as the links and bushings of the drivetrack chain and even the idler rollers themselves, begin to wear therebycreating slack in the drive track chain. In order to remove the slackfrom the drive track chain, it is necessary to increase the tension ofthe track chain. Such an increase in the tension is generallyaccomplished by manually injecting or otherwise inserting a material,such as grease, into the tension adjustment mechanism associated withthe undercarriage assembly.

The use of such tension adjustment mechanisms has a number of drawbacksassociated therewith. For example, manual injection of grease into theslack adjustment mechanism is a labor intensive task which can oftenlead to a decrease in the efficiency associated with operation of thework machine. Moreover, the drive track chain may inadvertently beoperated for a period of time with excessive slack therein. This is truesince the drive track chain may be operated with excessive slack fromthe point in time in which slack is first introduced into the trackchain until the point in time in which the tension is manuallyincreased. Such excessive slack may cause irregular wear of a number ofthe components associated with the undercarriage assembly. Moreover,such excessive slack may also cause the drive track chain to derailduring movement of the work machine thereby reducing the efficiency ofthe work machine due to the delays caused by repair of the undercarriageassembly.

In order to avoid the problems associated with excessive slack in thedrive track chain, heretofore designed undercarriage assemblies havebeen operated with relatively large amounts of tension on the drivetrack chain. However, operation of the undercarriage assembly with arelatively taut track chain during advancement of the work machineincreases the rate at which components associated with the undercarriageassembly wear thereby potentially reducing the useful life of theundercarriage assembly.

Moreover, with particular regard to excavators, it is generallydesirable to have the drive track chain relatively taut duringperformance of a digging or other type of work function in order toprevent the excavator from rolling back and forth within the interior ofthe drive track chain as a result of recoil forces generated duringperformance of the digging operation. Hence, a relatively high tensionlevel is typically maintained on the drive track chains of excavators atall times even though it is known that use of such a high tension levelincreases the rate at which components associated with the undercarriageassembly wear during advancement of the excavator.

What is needed therefore is a track tensioning assembly which overcomesone or more of the above-mentioned drawbacks.

DISCOURSE OF THE INVENTION

In accordance with a first embodiment of the present invention, there isprovided a valve assembly for controlling actuation of an actuator of atrack tensioning system for adjusting tension on a drive track chain.The valve assembly includes a valve housing defining (i) a masterchamber, (ii) a slave chamber, (iii) a fluid passage which fluidlycouples the master chamber to the slave chamber, and (iv) a fluid outletwhich fluidly couples the slave chamber to the actuator. The valveassembly also includes a master valve member positioned within themaster chamber. The master valve member (i) isolates the master chamberfrom the slave chamber when the master valve member is positioned in afirst master position, and (ii) places the master chamber in fluidcommunication with the slave chamber when the master valve member ispositioned in a second master position. The valve assembly furtherincludes a slave piston positioned within the slave chamber, the slavepiston (i) isolates the fluid outlet from the slave chamber when theslave piston is positioned in a first slave position, and (ii) placesthe fluid outlet in fluid communication with the slave chamber when theslave piston is positioned in a second slave position. Movement of themaster valve member from the first master position to the second masterposition causes movement of the slave piston from the first slaveposition to the second slave position. Movement of the slave piston fromthe first slave position to the second slave position causes actuationof the actuator, whereby tension on the drive track chain is adjusted bythe actuator.

In accordance with a second embodiment of the present invention, thereis provided an undercarriage assembly of a work machine. Theundercarriage assembly includes a pressurized hydraulic fluid source, adrive track chain, and an actuator mechanically coupled to the drivetrack chain so as to adjust tension on the drive track chain. Theundercarriage also includes a valve assembly for controlling actuationof the actuator. The valve assembly has (i) a valve housing defining (a)a fluid chamber, (b) a chamber inlet, and (c) a chamber outlet, (ii) apiston positioned within the fluid chamber so as to divide the fluidchamber into a spring chamber portion and a high-pressure chamberportion, and (iii) a biasing spring positioned in the spring chamberportion so as to exert a spring bias on an outer surface of the piston.The chamber inlet fluidly couples the hydraulic fluid source to thespring chamber portion. The chamber outlet fluidly couples thehigh-pressure chamber portion to the actuator. The piston has a firstfluid contact surface which is positioned in the spring chamber portion.The piston further has a second fluid contact surface which ispositioned in the high-pressure chamber portion. The first fluid contactsurface has a first diameter, (vi) the second fluid contact surface hasa second diameter. The first diameter is greater than the seconddiameter.

In accordance with a third embodiment of the present invention, there isprovided an undercarriage assembly of a work machine. The undercarriageassembly includes a first frame member and a second frame member whichis movable relative to the first frame member. The undercarriageassembly includes an actuator mechanically coupled to the second framemember so as to move the second frame member relative to the first framemember. The undercarriage assembly also includes an idler wheelrotatably secured to the second frame member. The undercarriage assemblyfurther includes a drive track chain which is advanced around the idlerwheel. Moreover, the undercarriage assembly includes a pressurizedhydraulic fluid source. The undercarriage assembly also includes a valveassembly for controlling actuation of the actuator. The valve assemblyhas (i) a valve housing defining (a) a fluid chamber, (b) a chamberinlet, and (c) a chamber outlet, (ii) a piston positioned within thefluid chamber so as to divide the fluid chamber into a spring chamberportion and a high-pressure chamber portion, and (iii) a biasing springpositioned in the spring chamber portion so as to exert a spring bias onan outer surface of the piston. The chamber inlet fluidly couples thehydraulic fluid source to the spring chamber portion. The chamber outletfluidly couples the high-pressure chamber portion to the actuator. Thepiston has a first fluid contact surface which is positioned in thespring chamber portion. The piston further has a second fluid contactsurface which is positioned in the high-pressure chamber portion. Thefirst fluid contact surface has a first diameter. The second fluidcontact surface has a second diameter. The first diameter is greaterthan the second diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a track-type tractor whichincorporates the features of the present invention therein;

FIG. 2 is an enlarged side elevational view of the undercarriageassembly of the tractor of FIG. 1;

FIG. 3 is cross sectional view of the frame assembly of theundercarriage assembly taken along the line 3—3 of FIG. 2, as viewed inthe direction of the arrows;

FIG. 4 is an enlarged fragmentary cross sectional view taken along theline 4—4 of FIG. 3, as viewed in the direction of the arrows (note thata number of the components shown in FIG. 4 are not shown in crosssection for clarity of description);

FIG. 5 is an enlarged fragmentary cross sectional view of the valvegroup assembly of the undercarriage assembly of FIG. 4;

FIG. 6 is a simplified block diagram of a portion of t tractor of FIG.1;

FIG. 7 is a side elevational view of an excavator which incorporates thefeatures of the present invention therein;

FIG. 8 is an enlarged, partially cutaway side elevational view of theundercarriage assembly of the excavator of FIG. 7;

FIG. 9 is a cross sectional view of the of the undercarriage assembly ofFIG. 8 which shows the track tensioning assembly positioned in itsretracted position (note that the control valve is schematically shownin FIG. 9 for clarity of description);

FIG. 10 is a view similar to FIG. 9, but showing the track tensioningassembly positioned in its extended position;

FIG. 11 is a schematic view of a second embodiment of a track tensioningassembly which incorporates the features of the present inventiontherein, note that in FIG. 11, the track tensioning assembly is shownpositioned in its retracted position;

FIG. 12 is a view similar to FIG. 11, but showing the track tensioningassembly positioned in its extended position;

FIG. 13 is a cross sectional view of the slack adjuster device of thetrack tensioning assembly of FIG. 11 which shows the slack adjusterassembly positioned in its increased-tension position (note that anumber of the components associated with the slack adjuster device arenot shown in cross section for clarity of description);

FIG. 14 is a view similar to FIG. 13 but showing the slack adjusterdevice positioned in its decreased-tension position; and

FIG. 15 is a schematic view of a third embodiment of a track tensioningassembly which incorporates the features of the present inventiontherein.

BEST MODE FOR CARRYING OUT THE INVENTION

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

Referring now to FIG. 1, there is shown a work machine such as atrack-type tractor 10 which is utilized to perform numerous workfunctions such as earth moving and ripping. The track-type tractor 10includes a number of work implements such as a hydraulically-poweredblade assembly 12 and a hydraulically-powered ripper 14. The track-typetractor 10 further includes an engine such as a diesel engine 60 forproviding the motive power for both advancing the tractor and operatingthe blade assembly 12 and the ripper 14.

The track-type tractor 10 also includes an undercarriage assembly 16.The undercarriage assembly 16 includes a frame assembly 18, a drivesprocket 20, a front idler wheel 22, a rear idler wheel 24, and a numberof midroller assemblies 26. An endless drive track chain 28 is driven bythe drive sprocket 20 so as to be advanced around each of the frontidler wheel 22, the rear idler roller 24, and the midroller assemblies26 thereby providing the motive power for the work machine. Inparticular, mechanical output from the engine 60 is transmitted to thedrive sprocket 20 via a transmission assembly (not shown) therebydriving the drive sprocket 20 so as to advance the drive track chain 28and hence the track-type tractor 10. It should be appreciated thatalthough only one side of the track-type tractor 10 is shown in FIG. 1,the other side of the tractor 10 would also include an undercarriageassembly 16 having similar components as those shown in FIG. 1 (e.g. aframe assembly 18, drive sprocket 20, idler wheels 22, 24, midrollerassemblies 26, and drive track chain 28).

The track-type tractor 10 also includes a cab 30. The cab 30 is providedto enclose or otherwise house the devices associated with the track-typetractor 10 which are utilized by an operator during operation of thetrack-type tractor 10. In particular, the cab 30 houses an operator seat(not shown) and a control assembly which includes, for example, asteering wheel and foot pedal assembly (not shown).

As shown in more detail in FIGS. 2-5, the frame assembly 18 of theundercarriage assembly 16 includes a front frame member 32 and a rearframe member 34. The front frame member 32 is movable relative to therear frame member 34. In particular, as shown in FIGS. 3 and 4, thefront frame member 32 is slidably secured to the rear frame member 34.More specifically, the front frame member 32 has a receiving slot 36defined therein. The receiving slot 36 receives an elongated extensiontab 38 that is defined in an intermediate frame member 40. As shown inFIG. 3, the intermediate frame member 40 is non-movably secured to therear frame member 34. The extension tab 38 is free to slide within thereceiving slot 36 thereby allowing the front frame member 32 to sliderelative to the intermediate frame member 40 and hence the rear framemember 34.

Movement of the front frame member 32 relative to the rear frame member34 causes corresponding movement of the idler wheels 22, 24 relative toone another. In particular, the front idler wheel 22 is rotatablysecured to the front frame member 32 via a yoke 180, whereas the rearidler wheel 24 is rotatably coupled to the rear frame member 34 via asupport arm 38. Hence, when the front frame member 32 is moved relativeto the rear frame member 34 in a forward direction (i.e. in the generaldirection of arrow 42 of FIG. 2), the front idler wheel 22 is likewisemoved relative to the rear frame member 34 and hence the rear idlerwheel 24 in the forward direction. Conversely, when the front framemember 32 is moved relative to the rear frame member 34 in a rearwarddirection (i.e. in the general direction of arrow 44 of FIG. 2), thefront idler wheel 22 is likewise moved relative to the rear frame member34 and hence the rear idler wheel 24 in the rearward direction.

As shown in FIG. 4, the frame assembly 18 further includes an actuator46 such as a hydraulically-powered piston assembly for moving the frontframe member 32 relative to the rear frame member 34. In particular, theactuator 46 includes a piston 48 which is mechanically secured to thefront frame member 32 via a coupling member 50. An end portion 52 of thepiston 48 is received into a fluid chamber 54 defined in a cylinderhousing 56. The cylinder housing 56 is in turn secured to the rear framemember 34. Changes in fluid pressure within the fluid chamber 54 causesmovement of the piston 48 and hence the front frame member 32 relativeto the rear frame member 34. In particular, an increase in fluidpressure within the fluid chamber 54 causes the piston 48 and hence thefront frame member 32 to be moved in the forward direction (i.e. in thegeneral direction of arrow 42 of FIGS. 2 and 4), thereby causing thefront idler wheel 22 to be likewise moved relative to the rear framemember 34 (and hence the rear idler wheel 24) in the forward direction.Conversely, a decrease in fluid pressure within the fluid chamber 54causes the piston 48 and hence the front frame member 32 to be moved inthe rearward direction (i.e. in the general direction of arrow 44 ofFIGS. 2 and 4), thereby causing the front idler wheel 22 to be likewisemoved relative to the rear frame member 34 (and hence the rear idlerwheel 24) in the rearward direction.

Fluid pressure within the fluid chamber 54 is controlled by a valvegroup 58. As shown in FIGS. 4 and 5, the valve group 58 includes ahousing 62 which has a fluid inlet 64 and a fluid outlet 66 definedtherein. The fluid inlet 64 is fluidly coupled to a pressurizedhydraulic fluid source 68 via a fluid line 70, whereas the fluid outlet66 is fluidly coupled to a hydraulic reservoir 72 via fluid line 74. Thepressurized hydraulic fluid source 68 may be any of the fluid circuitsassociated with the track-type tractor 10. As shall be discussed belowin greater detail, the design of the valve group 58 enables use of arelatively low pressure fluid source as the pressurized hydraulic fluidsource 68. For example, fluid pressure from the hydraulic transmission(not shown) of the track-type tractor 10 preferably functions as thepressurized hydraulic fluid source 68 thereby providing for a flow ofpressurized hydraulic fluid at relatively low pressure (e.g. 400 psi).It should be appreciated that use of a relatively low pressure sourcesuch as the hydraulic transmission eliminates the need to utilizehydraulic fluid pressure from high-pressure, highly-utilized hydraulicsystems such as the fluid circuits which power the work implements ofthe tractor 10 such as the blade assembly 12 and the ripper 14.

The valve housing 62 also has a master chamber 76 and a slave chamber 78defined therein. A first end portion 82 of the master chamber 76 isfluidly coupled to the fluid inlet 64 via a fluid passage 80. Hence,pressurized hydraulic fluid is advanced from the pressurized hydraulicfluid source 68 is advanced to the master chamber 76 via a fluid pathwhich includes the fluid line 70, the fluid inlet 64, and the fluidpassage 80. A second end portion 84 of the master chamber 76 is fluidlycoupled to the fluid outlet 66 via a fluid passage 86. Hence, hydraulicfluid is advanced from the master chamber 76 to the fluid reservoir 72via a fluid path which includes the fluid passage 86, the fluid outlet66, and the fluid line 74. Moreover, the master chamber 76 is fluidlycoupled to the slave chamber 78 via a fluid passage 88.

The valve group 58 includes a master valve assembly 90 having a mastervalve member 92, along with a slave valve assembly 94 having a slavepiston 96. As shown in FIG. 5, the master valve member 92 is positionedin the master chamber 76 and has three separate valve sections 98, 100,102. The master valve member 92 may be selectively positioned in orderto selectively provide for a flow of pressurized hydraulic fluid to theslave chamber 78. In particular, the master valve assembly 90 is anelectrically-actuated valve assembly having an electrically-actuatedsolenoid 104. Actuation of the solenoid 104 urges the master valvemember 92 leftwardly (as viewed in FIG. 5) into a position in which themaster chamber 76 is placed in fluid communication with the slavechamber 78. In particular, actuation of the solenoid 104 urges themaster valve member 92 leftwardly (as viewed in FIG. 5) so as to allowpressurized hydraulic fluid to flow through the fluid inlet 64, thefluid passage 80, the master chamber 76, the fluid passage 88, and intothe slave chamber 78. Note that when the master valve member 92 is urgedleftwardly in such a manner, the valve sections 100 and 102 preventhydraulic fluid from being advanced through a pair of branches 106, 108,respectively, of the fluid line 86 thereby isolating the master chamber76 from the reservoir 72.

Conversely, deactuation of the solenoid 104 urges the master valvemember 92 rightwardly (as viewed in FIG. 5) into a position in which thefirst portion 82 of the master chamber 76 is isolated from the slavechamber 78. In particular, deactuation of the solenoid 104 urges themaster valve member 92 rightwardly (as viewed in FIG. 5) into itsposition as shown in FIG. 5 in which the valve section 98 preventspressurized hydraulic fluid from flowing from the master chamber 76 tothe slave chamber 78 via the fluid passage 88. Note that when the mastervalve member 92 is urged rightwardly in such a manner, hydraulic fluidis allowed to advance through the branches 106, 108 of the fluid line 86thereby draining any fluid within the second end portion 84 of themaster chamber 76 to the reservoir 72.

The slave piston 96 is positioned in the slave chamber 78 therebydividing the slave chamber 78 into a spring chamber portion 110 and ahigh-pressure chamber portion 112. A biasing spring 114 is positioned inthe spring chamber portion 110 having a first end positioned in contactwith the valve housing 62 and a second end positioned in contact with anouter surface of the master piston 96. The biasing spring 114 isprovided to bias or otherwise urge the master piston 96 leftwardly (asviewed in FIG. 5) and into contact with a piston stop 116.

The slave piston 96 is preferably embodied as a stepped piston having afirst end portion 118 and a second end portion 120. The first endportion 118 is positioned within the spring chamber portion 110 of theslave chamber 78, whereas the second end portion 120 of the slave piston96 is positioned in the high-pressure chamber portion 112 of the slavechamber 78. As shown in FIG. 5, the diameter of the first end portion118 of the slave piston 96 is greater than the diameter of the secondend portion 120 of the slave piston 96. Such a configuration allows arelatively high output fluid pressure to be generated from a relativelysmall input fluid pressure. In particular, the first end portion 118 ofthe slave piston has a fluid contact surface 122, whereas the second endportion 120 of the slave piston has a fluid contact surface 124. Thesurface area of the fluid contact surface 122 is preferablyapproximately ten times greater than the surface area of the fluidcontact surface 122. Hence, fluid pressure created by the second endportion 120 of the slave piston 96 is approximately ten times greaterthan the fluid pressure exerted on the fluid contact surface 122 of thefirst end portion 118 of the slave piston 96. For example, if hydraulicfluid pressurized to 400 pounds-per-square-inch is exerted on the firstfluid contact surface 122 of the first end portion 118 of the slavepiston 96, the pressure of the hydraulic fluid in the high-pressurechamber portion 112 of the slave chamber 78 will be increased toapproximately 4,000 pounds-per-square-inch.

The slave chamber 78 also defines a fluid outlet 126 having a checkvalve assembly 128 positioned therein. The check valve assembly 128 hasa ball 130 which is biased into a valve seat 132 by a biasing spring134. The check valve assembly 128 is held in position within the valvehousing 62 by a threaded cap 136. The valve housing 62 also has a fluidport 138 defined therein. The fluid port 138 is fluidly coupled to afluid port 140 (see FIG. 4) defined in the cylinder housing 56 therebyplacing the high-pressure chamber portion 112 of the slave chamber 78 influid communication with the fluid chamber 54 of the actuator 46. Thecheck valve assembly 128 has a closed check position (as shown) in whichthe ball 130 is urged into contact with the valve seat 132 by the spring134 thereby isolating the fluid chamber 54 of the actuator 46 from thehigh-pressure chamber 112 of the slave chamber 78. The check valveassembly 128 also has an open check position in which the ball 130 isurged rightwardly (as viewed in FIG. 5) so as to allow fluid to advancefrom the high-pressure chamber portion 112 of the slave chamber 78 tothe fluid chamber 54 of the actuator 46. As shall be discussed below ingreater detail, the ball 130 may be urged rightwardly in order toposition the check valve assembly 128 into its open check position bythe presence of fluid pressure of a predetermined magnitude within thehigh-pressure chamber portion 112 of the slave chamber 78. The ball 130may also be urged rightwardly in order to position the check valveassembly 128 into its open check position by a contact protrusion 176defined in the second end portion of the slave piston 96 during movementof the slave piston 96.

The valve group 58 also includes a pair of electrically-actuated controlvalves 142, 144. The control valve 142 selectively allows pressurizedhydraulic fluid to be advanced from the pressurized hydraulic fluidsource 68 (see FIG. 4) to the high-pressure chamber portion 112 of theslave chamber 78. In particular, the inlet of the control valve 142 isfluidly coupled to the fluid inlet 64 of the valve housing 62, whereasthe outlet of the control valve 142 is fluidly coupled to thehigh-pressure chamber portion 112 of the slave chamber 78 via a fluidpassage 146. The control valve 142 has a closed control position (asshown in FIG. 5) and an open control position. In particular, thecontrol valve 142 includes an electrically-actuated solenoid 148 which,upon actuation thereof, causes the control valve 142 to shift from itsclosed control position to its open control position. Deactuation of thesolenoid 148 cause the control valve 142 to shift from its open controlposition to its closed control position. It should be appreciated thatwhen the control valve 142 is positioned in its closed control position,the high-pressure chamber portion 112 of the slave chamber 78 isisolated from the fluid inlet 64 (and hence the pressurized hydraulicfluid source 68), whereas when the control valve 142 is positioned inits open control position, the high-pressure chamber portion 112 of theslave chamber 78 is fluidly coupled to the fluid inlet 64 (and hence thepressurized hydraulic fluid source 68).

The control valve 144 selectively allows hydraulic fluid to be exhaustedor otherwise drained from the high-pressure chamber portion 112 of theslave chamber 78 to the reservoir 72. In particular, the inlet of thecontrol valve 144 is fluidly coupled to the high-pressure chamberportion 112 of the slave chamber 78 via a fluid passage 150, whereas theoutlet of the control valve 144 is fluidly coupled to the fluid outlet66 of the valve housing 62. The control valve 144 has a closed controlposition (as shown in FIG. 5) and an open control position. Inparticular, similarly to the control valve 142, the control valve 144includes an electrically-actuated solenoid 152 which, upon actuationthereof, causes the control valve 144 to shift from its closed controlposition to its open control position. Deactuation of the solenoid 152cause the control valve 142 to shift from its open control position toits closed control position. It should be appreciated that when thecontrol valve 144 is positioned in its closed control position, thehigh-pressure chamber portion 112 of the slave chamber 78 is isolatedfrom the fluid outlet 66 (and hence the reservoir 72), whereas when thecontrol valve 144 is positioned in its open control position, thehigh-pressure chamber portion 112 of the slave chamber 78 is fluidlycoupled to the fluid outlet 66 (and hence the reservoir 72).

As shown in FIG. 4, a position sensor 154 is positioned within the frameassembly 16 in order to monitor the position of a number ofundercarriage components relative to one another. In particular, theposition sensor 154 is provided to sense the position of the front framemember 32 relative to the rear frame member 34. As shall be discussedbelow in greater detail, the position of the front frame member 32relative to the rear frame member 34 may be utilized to provide “closedloop” control of the tension on the drive track chain 28.

The position sensor 154 may be provided as any type of sensor which iscapable of sensing the position of the front frame member 32 relative tothe rear frame member 34. In an exemplary embodiment, the positionsensor 154 is provided as a linear displacement transducer which sensesthe linear distance between a sensing location associated with the rearframe member 34 and a sensing location associated with the front framemember 32. For example, as shown in FIG. 4, the position sensor 154 maybe utilized to sense a linear distance D between the location on thecylinder housing 56 at which the sensor 154 is secured (e.g. a sensinglocation associated with the rear frame member 34) and a designatedsensing location 156 associated with the front frame member 32. Onecommercially available sensor which is particularly useful as theposition sensor 154 of the present invention is a Series BTL-2 LinearDisplacement Transducer which is commercially available Balluffe,Incorporated of Florence, Ky.

Referring now to FIG. 6, there is shown a simplified block diagram of atrack tensioning control system 158 of the track-type tractor 10. Asshown in FIG. 6, the master valve assembly 90, the inlet control valve142, the outlet control valve 144, and the position sensor 154 are eachelectrically coupled to a processing unit such as a controller 160. Thecontroller 160 may be a dedicated controller for controlling thecomponents shown in FIG. 6, or may alternatively be integrated intoanother controller associated with the track-type tractor 10 such as theengine controller (not shown), transmission controller (not shown), orimplement controller (not shown).

The controller 160 includes electrical components commonly found inother work machine controllers such as a microprocessor 162, a memorydevice 164, and an interface circuit 166. The interface circuit 166converts the output signals from the position sensor 154 into a signalwhich is suitable for presentation to an input of the microprocessor162. In particular, an analog-to-digital (A/D) converter (not shown)associated with the interface circuit 166 converts the analog voltage orother type of output signal generated by the position sensor 154 into adigital value for use by the microprocessor 162. It should beappreciated that the magnitude of the analog voltage generated by theposition sensor 154 is indicative of the linear distance D between thefront frame member 32 and the rear frame member 34.

The interface circuit 166 also converts output signals generated by themicroprocessor 162 into a signal which is suitable for use by thesolenoids 104, 148, 152 associated with the valves 90, 142, 144,respectively. In particular, the interface circuit 166 converts theoutput signals from the microprocessor into an analog actuation pulsewhich actuates the solenoids 104, 148, 152 thereby positioning thevalves 90, 142, 144, respectively, into their respective actuatedpositions described above. It should be further appreciated that theinterface circuit 166 may be embodied as a discrete device or number ofdevices, or may be integrated into the microprocessor 162.

The memory device 164 is provided to store the code or set ofinstructions which are executed by the controller 160 during operationof the track tensioning control system 158. Moreover, operationparameters may also be stored in the memory device 164. The memorydevice 164 may be embodied as any known memory device such as RAM and/orROM devices.

As shown in FIG. 6, the position sensor 154 is electrically coupled tothe controller 160 via a signal line 168. Hence, output signalsgenerated by the position sensor 154 are communicated to the controller160 via the signal line 168. As discussed above, such output signals maybe generated and thereafter communicated by the position sensor innumerous forms. For example, the position sensor 154 may generate outputsignals in the form of an analog DC voltage or in the form of a signalutilizing current-to-pulse signal timing.

The master valve assembly 90 is also electrically coupled to thecontroller 160. In particular, the solenoid 104 of the master valveassembly 90 is electrically coupled to the controller 160 via a signalline 170. Hence, the controller 160 generates output signals in the formof actuation pulses on the signal line 170 which actuate the solenoid104 thereby urging the master valve member 92 leftwardly (as viewed inFIG. 5) into a position in which the master chamber 76 is placed influid communication with the slave chamber 78 so as to allow pressurizedhydraulic fluid to flow through the fluid inlet 64, the fluid passage80, the master chamber 76, the fluid passage 88, and into the slavechamber 78. As discussed above, when the master valve member 92 is urgedleftwardly in such a manner, the valve sections 100 and 102 preventhydraulic fluid from being advanced through a pair of branches 106, 108,respectively, of the fluid line 86 thereby isolating the master chamber76 from the reservoir 72.

When the controller 160 ceases to generate an actuation pulse on thesignal line 170, the solenoid 104 is deactuated. As described above,deactuation of the solenoid 104 urges the master valve member 92rightwardly (as viewed in FIG. 5) into a position in which the masterchamber 76 is isolated from the slave chamber 78. Note also that whenthe master valve member 92 is urged rightwardly in such a manner,hydraulic fluid is allowed to advance through the branches 106, 108 ofthe fluid line 86 thereby draining any fluid within the second endportion 84 of the master chamber 76 to the reservoir 72.

Similarly, the inlet control valve 142 is electrically coupled to thecontroller 160. In particular, the solenoid 148 of the inlet controlvalve 142 is electrically coupled to the controller 160 via a signalline 172. Hence, the controller 160 generates output signals in the formof actuation pulses on the signal line 172 which actuate the solenoid148 thereby shifting the inlet control valve 142 from its closed controlposition in which the high-pressure chamber portion 112 of the slavechamber 78 is isolated from the fluid inlet 64 to its open controlposition in which the high-pressure chamber portion 112 of the slavechamber 78 is fluidly coupled to the fluid inlet 64.

When the controller 160 ceases to generate an actuation pulse on thesignal line 172, the solenoid 148 is deactuated. As described above,deactuation of the solenoid 148 causes the control valve 142 to shiftfrom its open control position to its closed control position therebyisolating the high-pressure chamber portion 112 of the slave chamber 78from the fluid inlet 64.

Moreover, the outlet control valve 144 is electrically coupled to thecontroller 160. In particular, the solenoid 152 of the outlet controlvalve 144 is electrically coupled to the controller 160 via a signalline 174. Hence, the controller 160 generates output signals in the formof actuation pulses on the signal line 174 which actuate the solenoid152 thereby shifting the outlet control valve 144 from its closedcontrol position in which the high-pressure chamber portion 112 of theslave chamber 78 is isolated from the fluid outlet 66 to its opencontrol position in which the high-pressure chamber portion 112 of theslave chamber 78 is fluidly coupled to the fluid outlet 66.

When the controller 160 ceases to generate an actuation pulse on thesignal line 172, the solenoid 148 is deactuated. As described above,deactuation of the solenoid 148 causes the control valve 142 to shiftfrom its open control position to its closed control position therebyisolating the high-pressure chamber portion 112 of the slave chamber 78from the fluid outlet 66.

The controller 160 communicates with each of the position sensor 154,the master valve assembly 90, the inlet control valve 142, and theoutlet control valve 144 in order to maintain a desired level of tensionon the drive track chain 28. In particular, the controller 160 initiallyexecutes a “zeroing” routine in which the controller 160 causessubstantially all of the slack to be removed from the drive track chain28. In order to accomplish this, the controller 160 generates an outputsignal on the signal line 172 so as to actuate the solenoid 148 of theinlet control valve 142 thereby causing pressurized hydraulic fluid tobe advanced into the high-pressure chamber portion 112 of the slavechamber 78 from the fluid inlet 64. Thereafter, the controller 160generates an output signal on the signal line 170 so as to actuate thesolenoid 104 associated with the master valve assembly 90 therebycausing pressurized hydraulic fluid to be advanced into the springchamber portion 110 of the slave chamber 78.

The presence of pressurized hydraulic fluid in the spring chamberportion 110 of the slave chamber 78 urges the slave piston 96rightwardly (as viewed in FIG. 5) thereby increasing fluid pressure inthe high-pressure chamber portion 112 of the slave chamber 78. Oncefluid pressure in the high-pressure chamber portion 112 of the slavechamber 78 has increased to a predetermined level, the ball 130 of thecheck valve assembly 128 is urged off of the valve seat 132 therebycausing pressurized hydraulic fluid to be advanced out the fluid port138 and into the fluid chamber 54 of the actuator 46 via the fluid port140 (see FIG. 4). The presence of the additional hydraulic fluidincreases the fluid pressure within the fluid chamber 54 thereby urgingthe piston 48 of the actuator 46 in the forward direction (i.e. in thegeneral direction of arrow 42 of FIGS. 2 and 4). Such forward movementof the piston 48 likewise urges the front frame member 32 and hence thefront idler wheel 22 in the forward direction (i.e. in the generaldirection of arrow 42 of FIGS. 2 and 4) thereby increasing tension onthe drive track chain 28. It should be appreciated that theabove-described procedure may be repeated until substantially all of theslack has been removed from the drive track chain 28. It should beappreciated that numerous techniques may be utilized to determine whensubstantially all of the slack has been removed from the drive trackchain 28. For example, visual inspection of the drive track chain 28 maybe performed in order to determine when the track chain 28 is taut orotherwise has substantially all of the slack removed therefrom.Moreover, a pressure sensor may be utilized to determine when fluidpressure within the fluid chamber 54 increases to a pressure level thatis indicative of substantially all of the slack having been removed fromthe drive track chain 28. In addition, the position sensor 154 may beutilized to determine if subsequent additions of hydraulic fluid intothe fluid chamber 54 are unable to further extend the piston 48 (i.e.unable to further move the front frame member 32 relative to the rearframe member 34) thereby indicating that substantially all of the slackhas been removed from the drive track chain 28.

In any event, once the controller 160 has established a “zero position”(i.e. substantially all of the slack has been removed from the drivetrack chain 28), a predetermined amount of hydraulic fluid is thenremoved from the fluid chamber 54 in order to retract or otherwise movethe piston 48 in a rearward direction (i.e. in the general direction ofarrow 44 of FIGS. 2 and 4) by a predetermined distance. In particular,the controller 160 generates an output signal on the signal line 174 inorder to actuate the solenoid 152 associated with the outlet controlvalve 144 thereby placing the high-pressure chamber portion 112 of theslave chamber 78 in fluid communication with the fluid outlet 66 (andhence the fluid reservoir 72). The controller 160 also generates anoutput signal on the signal line 170 so as to actuate the solenoid 104associated with the master valve assembly 90 thereby causing pressurizedhydraulic fluid to be advanced into the spring chamber portion 110 ofthe slave chamber 78. The presence of pressurized hydraulic fluid in thespring chamber portion 110 of the slave chamber 78 urges the slavepiston 96 rightwardly (as viewed in FIG. 5) thereby causing the contactprotrusion 176 to urge the ball 132 of the check valve assembly 128 offof its valve seat 132. Once the ball 130 is unseated, hydraulic fluid isallowed to flow out of the fluid chamber 54 of the actuator 46, throughthe ports 140, 138, and out the fluid outlet 66 thereby retracting orotherwise moving the piston 48 in a rearward direction (i.e. in thegeneral direction of arrow 44 of FIGS. 2 and 4) thereby decreasingtension on the drive track chain 28. Once the position sensor 154detects that the front frame member 32 has been moved rearwardly by thepredetermined distance, the controller 160 ceases to generate outputsignals on the signal lines 170, 174 thereby ceasing retraction of thepiston 48 of the actuator 46 so as to position the front frame member 32(and hence the front idler wheel 22) in a “target position”. It shouldbe appreciated that retraction of the piston 48 by the predetermineddistance creates a corresponding predetermined amount of slack in thedrive track chain 28.

Thereafter, the position sensor 154 is utilized to maintain the frontframe member 32 (and hence the front idler wheel 22) in the targetposition. In particular, if the position sensor 154 detects that thelinear distance D from the front frame member 32 to the rear framemember 34 decreases below a predetermined distance value with respect tothe target position of the front frame member 32 thereby indicating thattension on the drive track chain has decreased, the controller 160generates output control signals which control operation of the mastervalve assembly 90 and the inlet control valve 142 so as to increasefluid pressure in the fluid chamber 54 of the actuator 48 in the mannerdescribed above so as to move the front frame member 32 in the forwarddirection (i.e. in the general direction of arrow 42 of FIGS. 2 and 4)so as to incrementally increase tension on the drive track chain 28.Once the front frame member 32 has been advanced back to the targetposition (as sensed by the position sensor 154) the controller 160ceases to increase fluid pressure in the fluid chamber 54 therebyceasing forward advancement of the piston 48.

Conversely, if the position sensor 154 detects that the linear distanceD from the front frame member 32 to the rear frame member 34 increasesabove the predetermined distance value with respect to the targetposition of the front frame member 32 thereby indicating that tension onthe drive track chain has increased, the controller 160 generates outputcontrol signals which control operation of the master valve assembly 90and the outlet control valve 144 so as to incrementally decrease fluidpressure in the fluid chamber 54 of the actuator 48 in the mannerdescribed above so as to move the front frame member 32 in the rearwarddirection (i.e. in the general direction of arrow 44 of FIGS. 2 and 4)so as to decrease tension on the drive track chain 28. Once the frontframe member 32 has been advanced back to the target position (as sensedby the position sensor 154) the controller 160 ceases to decrease fluidpressure in the fluid chamber 54 thereby ceasing rearward advancement ofthe piston 48.

It should be appreciated that such “closed loop” control of the tensionon the drive track chain 28 prevents the track-type tractor 10 frombeing operated for a period of time with an undesirable amount oftension (either too high or too low) in the drive track chain 28 therebyincreasing the efficiency tractor 10 while also increasing the usefullife of the components associated with the undercarriage assembly 16.Moreover, it should be appreciated that the controller 160 may also beconfigured to automatically re-execute the “zeroing” procedure atpredetermined intervals so as to account for normal wear in thecomponents associated with the drive track chain 28.

Moreover, it should be appreciated that a pressure relief valve (notshown) may be fluidly interposed between the fluid port 138 of the valvegroup 58 and the fluid port 140 of the actuator 46 in order to provide arecoil function to the undercarriage assembly 16. In particular, if arock or the like is ingested by the undercarriage assembly 16 duringadvancement of the track-type tractor 10, the front idler wheel 22 isurged or otherwise moved rearwardly (i.e. in the general direction ofarrow 44 of FIGS. 2 and 4) thereby increasing fluid pressure in thefluid chamber 54 of the actuator 46. Once fluid pressure within thefluid chamber 54 is greater in magnitude than the relief setting of thepressure relief valve (e.g. 6,000 pounds-per-square-inch), hydraulicfluid within the fluid chamber 54 is exhausted to the reservoir via thepressure relief valve thereby allowing the piston 48 (and hence thefront frame member 32 and the front idler wheel 22) to be urged orotherwise moved in the rearward direction (i.e. in the general directionof arrow 44 of FIGS. 2 and 4) thereby providing relief or slack in thedrive track chain 28. It should be appreciated that such relief in thedrive track chain 28 facilitates expulsion of the rock from theundercarriage assembly 16.

Once the rock has been expelled from the undercarriage assembly 16, thefront frame member 32 is returned to its previous target positionthereby returning the drive track chain 28 to its previous tensionlevel. In particular, the controller 160 controls actuation of thevalves 90, 142, and 144 based on output from the position sensor 154 inorder to return the front frame member 32 (and hence the front idlerwheel 22) to its previous target position thereby returning the drivetrack chain 28 to its previous tension level. It should be appreciatedthat the controller 160 may alternatively be configured to execute the“zeroing procedure” so as to reset the target position of the frontframe member 32 after each recoil event.

Referring now to FIG. 7, there is shown another type of work machinesuch as a hydraulic excavator 210 which is utilized to perform numerouswork functions such as digging and material movement. The excavator 210includes a number of work implements such as a hydraulically-poweredbucket assembly 212 which is secured to an end of a boom assembly 214having a boom arm 216 and a stick assembly 218. The excavator 210further includes an engine such as a diesel engine 220 for providing themotive power for both advancing the excavator 210 and operating thebucket 212 and boom assembly 214.

The excavator 210 also includes an undercarriage assembly 226. Theundercarriage assembly 226 includes a frame assembly 228, a drivesprocket 230, a front idler wheel 232, and a number of midrollerassemblies 236. An endless drive track chain 238 is driven by the drivesprocket 230 so as to be advanced around the front idler wheel 232 andeach of the midroller assemblies 236 thereby providing the motive powerfor advancing the excavator 210. In particular, mechanical output fromthe engine 220 is transmitted to the drive sprocket 230 via a hydraulicdrive system 240 having a number of hydraulic drive motors 304 (seeFIGS. 9 and 10) which drive the drive sprocket 230 so as to advance thedrive track chain 238 and hence the excavator 210. It should beappreciated that although only one side of the excavator 210 is shown inFIG. 7, the other side of the excavator 210 would also include anundercarriage assembly 226 having similar components as those shown inFIG. 7 (e.g. a frame assembly 228, drive sprocket 230, front idler wheel232, midroller assemblies 236, and drive track chain 238).

The excavator 210 also includes a cab 240. The cab 240 is provided toenclose or otherwise house the devices associated with the excavator 210which are utilized by an operator during operation of the excavator 210.In particular, the cab 240 houses an operator seat (not shown) and anumber of control devices 242 such as, for example, a control leverassembly 466 and a foot pedal assembly 246 (see FIG. 15).

As shown in more detail in FIGS. 8-10, the undercarriage assembly 226includes a track tensioning assembly 248. The track tensioning assembly248 includes a cylinder assembly 250 having a yoke 252 secured thereto.As shown in FIG. 8, the front idler wheel 232 is rotatably coupled tothe yoke 252. Movement of the yoke 252 and hence the front idler wheel232 in a forward direction (i.e. in the general direction of arrow 254of FIG. 8) increases tension of the drive track chain 238. Conversely,movement of the yoke 252 and hence the front idler wheel 232 in arearward direction (i.e. in the general direction of arrow 256 of FIG.8) decreases tension of the drive track chain 238.

As shown in FIGS. 9 and 10, the cylinder assembly 250 includes a mainhousing 258 having a main chamber 260 defined therein. The main chamber260 includes a recoil subchamber 262 which has a recoil piston 264positioned therein. As shall be discussed below in greater detail, therecoil piston 264 provides a recoil function to the undercarriageassembly 226. In particular, if a rock or the like is ingested by theundercarriage assembly 226 during advancement of the excavator 210, useof the recoil piston 264 facilitates expulsion of the rock from theundercarriage assembly 226 without damage to the components associatedwith the undercarriage assembly 226 such as the drive track chain 238.

The recoil piston 264 has a central passage 266 defined therein. Amaster piston assembly 268 is secured within the central passage 266.The master piston assembly 268 includes a housing 270 having a masterchamber 272 and an outlet port 274 defined therein. A master piston 276is positioned within the master chamber 272 in order to translate backand forth therein. The master piston 276 has a central passage 278defined therein. A check valve assembly 280 is positioned within thecentral passage 278 of the master piston 276.

The cylinder assembly 250 further includes a slave piston 280 having ahead end portion 282 positioned within the main chamber 260 and a rodend portion 284 which extends out of a sealed opening 286 defined in themain housing 258. As shown in FIG. 9, the rod end portion 284 is securedto the yoke 252 and hence the idler wheel 232. Therefore, as the headend portion 282 of the slave piston 280 is moved in the forwarddirection (i.e. in the general direction of arrow 254 of FIGS. 8-10),the yoke 252 and hence the front idler wheel 232 are likewise moved inthe forward direction thereby increasing tension of the drive trackchain 238. Conversely, as the head end portion 282 of the slave piston280 is moved in the rearward direction (i.e. in the general direction ofarrow 256 of FIGS. 810), the yoke 252 and hence the front idler wheel232 are likewise moved in the rearward direction thereby decreasingtension of the drive track chain 238.

The track tensioning assembly 248 further includes a control valveassembly 288 and a nitrogen-charged fluid accumulator 290. Theaccumulator 290 is coupled to a fluid port 292 of the control valveassembly 288 via a fluid line 294. The recoil subchamber 262 is coupledto a fluid port 300 of the control valve assembly 288 via a fluid line302, whereas the master chamber 272 of the master piston assembly 268 iscoupled to a fluid port 296 of the control valve assembly 288 via afluid line 298. A fluid outlet port 306 of the control valve assembly288 is fluidly coupled to a fluid reservoir 308 via a drain line 310.

A pilot fluid port 312 is fluidly coupled to one or more componentsassociated with the hydraulic drive system 240 of the excavator 210 viaa fluid line 314. In particular, as described above, the hydraulic drivesystem 240 of the excavator 210 includes a number of hydraulic drivemotors 304 for driving the drive sprockets 230 of the undercarriageassemblies 226. Moreover, the hydraulic drive system 240 includes ahydraulically-deactuated parking brake 316. The parking brake 316includes a retaining spring (not shown) which retains the excavator 210in a relatively stationary position when the parking brake 316 isactuated. The retaining spring is released by a flow of pressurizedhydraulic fluid into a spring chamber (not shown) which houses theretaining spring so as to allow the excavator 210 to be advanced fromone location to another.

The hydraulic drive system 240 also includes a switching valve assembly318 which is operatively coupled to the operator control devices 242located within the cab 240. The switching valve assembly 318 selectivelyprovides for a flow of pressurized hydraulic fluid to the componentsassociated with the hydraulic drive system 240 and the implementsassociated with the excavator 210 based on manipulation of the controldevices 242 by the operator of the excavator 210. In particular, thehydraulic excavator 210 includes a main fluid supply circuit 488 (seeFIG. 15) which includes the fluid components necessary to supplypressurized hydraulic fluid the drive system 240 and the hydraulicimplements associated with the excavator 210. Amongst other functions,the switching valve assembly 318 directs pressurized hydraulic fluidfrom the main fluid supply circuit 488 to both the drive system 240 andan implement supply circuit 464 (see FIG. 15). Hence, if the operatormanipulates one of the control devices in order to advance the excavator210, fluid is directed from the main fluid supply circuit 488 to thecomponents associated with the drive system 240 by the switching valveassembly 318. Conversely, if the operator manipulates one of the controldevices in order to operate a work implement such as the bucket 212,fluid is directed 10 from the main fluid supply circuit 488 to theimplement supply circuit 464 by the switching valve assembly 318. As aparticular example, if the operator manipulates one of the controldevices 242 in order to release the parking brake 316, the switchingvalve assembly 318 directs pressurized hydraulic fluid to the springchamber associated with the parking brake 316 so as to release the brake316. Similarly, if the operator manipulates one of the control devices242 such as the foot pedal 246 (see FIG. 15) in order to advance theexcavator 210, the switching valve assembly 318 directs pressurizedhydraulic fluid to the appropriate drive motors 304.

The control valve assembly 288 of the track tensioning assembly 248 ispositionable in a number of control positions based on the activitybeing performed by the excavator 210 in order to selectively increase ordecrease tension on the drive track chain 238. Such adjustment of tracktension provides the excavator 210 of the present invention withnumerous advantages over heretofore designed excavators. For example,when the excavator 210 is performing a work function such as a diggingfunction or a material handling function, the track tensioning system248 of the present invention increases tension on the drive track chain238 so as to remove substantially all of the slack therefrom. Removal ofsubstantially all of the slack from the drive track chain 238 preventsthe undercarriage assembly 226 from rolling back and forth within theinterior of the drive track chain 238. However, the track tensioningsystem 248 of the present invention decreases tension on the drive trackchain 238 when the excavator 210 is being advanced in order to reducewear on the components associated with the undercarriage assembly 226thereby increasing the useful life of the excavator 210.

It should be appreciated that actuation or deactuation of the hydraulicdrive system 240 may be monitored in order to determine the activitybeing performed by the excavator 210. For example, an increase in fluidpressure in the fluid supply lines (not shown) which supply thehydraulic motors 304 is indicative of advancement (i.e. movement) of theexcavator 210. Similarly, a decrease in fluid pressure in the fluidsupply lines which supply the hydraulic motors 304 is indicative of theexcavator 210 being maintained in a stationary position (i.e. not movingor otherwise being advanced) such as when the excavator 210 isperforming a work function (e.g. a digging function or material handlingfunction). Moreover, an increase in fluid pressure in the fluid supplyline (not shown) which supplies the hydraulically-deactuated parkingbrake 316 is indicative of the operator preparing to advance theexcavator 210. Similarly, fluid pressure in any of the fluid linesassociated with the switching valves associated with the switching valveassembly 318 may also be monitored to determine if the excavator 210 isbeing advanced or being maintained in a stationary position. Moreover,an increase in fluid pressure in the fluid supply lines (not shown)which supply hydraulic fluid to fluid cylinders associated with the boomassembly 214 and the bucket 212 (see FIG. 7) is indicative of theexcavator 210 being operated to perform a work function.

From the above discussion, it should be appreciated that the fluid line314 may be fluidly coupled to any one of numerous components associatedwith the hydraulic drive system 240 (or the implement fluid supplycircuit 464) in order to communicate changes in hydraulic pressure tothe pilot fluid port 312 of the control valve assembly 288. It should beappreciated that such changes in hydraulic pressure are indicative ofthe hydraulic drive system 240 being switched between its actuated modeof operation in which the drive system 240 causes advancement of theexcavator 210 from one location to another and its deactuated mode ofoperation in which the drive system 240 does not advance the excavator210. In an exemplary embodiment, the fluid line 314 is fluidly coupledto the fluid inlet line of the parking brake 316. In such aconfiguration, the control valve assembly 288 is placed in anincrease-tension position (as shown in FIG. 10) when relatively lowfluid pressure is sensed in the fluid inlet line of the parking brake316. However, if an increase in fluid pressure is sensed in the fluidinlet line of the parking brake 316 thereby indicating that the brake316 is being released, the control valve assembly 288 is positioned inits decrease-tension position, as shown in FIG. 9.

As shown in FIG. 10, when the control valve assembly 288 is positionedin its increase-tension position, pressurized hydraulic fluid isadvanced from the accumulator 290 to each of the fluid lines 298, 302,and therefore into the recoil subchamber 262 and the master chamber 272.Presence of pressurized hydraulic fluid in the recoil subchamber 262urges the recoil piston 264 leftwardly (as viewed in FIGS. 9 and 10)against a stop 320. Pressurized hydraulic fluid in the master chamber272 is advanced through the central passage 278 define in the masterpiston 276, through the check valve 280, and out the outlet port 274 ofthe housing 270. The check valve 280 maintains a small differentialfluid pressure between the master chamber 272 and the outlet port 274thereby urging the master piston 276 leftwardly (as viewed in FIGS. 9and 10).

Such leftward movement of the master piston 276 increases fluid pressurein the main chamber 260 so as to urge the head end portion 282 of theslave piston 280 in the forward direction (i.e. in the general directionof arrows 254 of FIGS. 8-10) thereby likewise moving the front idlerwheel 232 in the forward direction. Pressurized hydraulic fluid from thefluid accumulator 290 will continue to urge the idler wheel 232 in theforward direction until substantially all of the slack is removed fromthe drive track chain 238 (i.e. the track is taut) or until the masterpiston 276 is advanced into contact with a stop 322. It should beappreciated that if the accumulator 290 is depleted of hydraulic fluidwithout substantially all of the slack being removed from the drivetrack chain 238, the above procedure may be cycled through again inorder to produce such a result.

It should be appreciated that increasing tension on the drive trackchain 238 during a digging operation facilitates operation of theexcavator 210. In particular, by removing substantially all of the slackfrom the drive track chain 238, the excavator 210 is less likely to rollback and forth within the inner is portion of the drive track chain 238.Moreover, as shown in FIG. 10, a pair of check valves 324, 326 withinthe control valve assembly 288 are provided to minimize, if noteliminate, movement of the components associated with the tracktensioning assembly 248 (and hence the front idler wheel 232) due toimplement forces generated by the boom assembly 214 and the bucket 212during operation thereof. This further eliminates undesirable movementof the undercarriage assembly 226 during performance of a work function.

As described above, if an increase in fluid pressure is sensed in thefluid inlet line of the parking brake 316 thereby indicating that thebrake 316 is being released, the control valve assembly 288 ispositioned in its decrease-tension position, as shown in FIG. 9. Whenthe control valve assembly 288 is positioned in its decrease-tensionposition, pressurized hydraulic fluid continues to be advanced from theaccumulator 290, along with fluid pressure from the drive system 240, tothe fluid line 302 and hence into the recoil subchamber 262. As before,presence of pressurized hydraulic fluid in the recoil subchamber 262urges the recoil piston 264 leftwardly (as viewed in FIGS. 9 and 10)against the stop 320.

However, when the control valve assembly 288 is positioned in itsdecrease-tension position, hydraulic fluid within the master chamber 272is exhausted or otherwise vented to the reservoir 308 thereby allowingpressurized fluid within the fluid outlet 274 and the main chamber 260(which is isolated from the reservoir 308 by the check valve 280) tourge the master piston 276 in a rightwardly direction (as viewed inFIGS. 9 and 10) until movement of the master piston 276 is stopped by astop 328.

Such rightward movement of the master piston 276 decreases fluidpressure in the main chamber 260 so as to urge the head end portion 282of the slave piston 280 in the rearward direction (i.e. in the generaldirection of arrow 256 of FIGS. 8-10) thereby likewise moving the frontidler wheel 232 in the rearward direction. The distance in which theidler wheel 232 is retracted (i.e. moved in the general direction ofarrow 256 of FIGS. 8-10) corresponds to the stroke length of the masterpiston 276. In an exemplary embodiment, a desirable traveling tracktension level is achieved by retracting the front idler wheel 232approximately ten millimeters from the point at which substantially allof the slack has been removed from the drive track chain 238 (i.e. thepoint at which the front idler 232 is positioned during performance of awork function). Hence, the master piston assembly 268 is configured suchthat the master piston 276 has a stroke length which causes the frontidler to be retracted approximately ten millimeters when the masterpiston 276 is urged rightwardly against the stop 328.

It should be appreciated that the hydraulic fluid lost during venting ofthe master chamber 272 is replaced in the accumulator 290 by use ofhydraulic fluid from the drive system 240. In particular, a pressureregulation valve 330 in the control valve assembly 288 provides fluidpressure at the fluid port 292 (via a check valve 332) which is greaterthan the pressure maintained by the accumulator 290. This increases thepressure in the accumulator 290 by forcing additional fluid thereinthereby replacing the fluid which was lost during venting of the masterchamber 272. In an exemplary embodiment, the pressure regulation valve330 maintains fluid pressure from the drive system 240 at 2,000pounds-per-square-inch, whereas the accumulator 290 provides fluidpressure at 1,875 pounds-per-square-inch. Hence, when fluidly coupled tothe drive system 240, fluid pressure from the drive system 240 (2,000psi) is greater than fluid pressure within the accumulator 290 (1,875psi) thereby forcing additional fluid into the accumulator 290. Itshould be appreciated that when the control valve assembly 288 isswitched back to its increase-tension position (see FIG. 10) the storedfluid pressure in the accumulator 290 urges the master piston 276leftwardly (as viewed in FIGS. 9 and 10) thereby dropping fluid pressurewithin the accumulator back to its normal pressure (e.g. back to 1,875psi from 2,000 psi).

As described above, such loosening of the drive track chain 238 prior toadvancement of the excavator 210 provides numerous advantages overheretofore designed excavators. For example, by loosening or otherwisedecreasing tension on the drive track chain 238 by a predeterminedamount prior to advancement of the excavator 210, wear on componentsassociated with the undercarriage assembly 226 is reduced therebyincreasing the efficiency and even the useful life of the excavator 210.

Moreover, it should be appreciated that the design of the tracktensioning system 248 provides a recoil function to the undercarriageassembly 226 thereby eliminating the need for a separate recoil assemblysuch as a spring or the like. In particular, a combination of a checkvalve 334 and pressure relief valve 336 allows for selective movement ofthe recoil piston 264 in the event that a rock or the like is ingestedby the undercarriage assembly 226 during advancement of the excavator210. Such movement of the recoil piston 264 causes correspondingmovement of the front idler wheel 232. In particular, if duringadvancement of the excavator 210, a rock or the like is ingested by theundercarriage assembly 226 thereby urging the idler wheel 232 and hencethe piston head end 282 of the slave piston rearwardly (i.e. in thegeneral direction of arrow 256 of FIGS. 8-10), fluid pressure is exertedon a first end 338 of the recoil piston 264 thereby increasing fluidpressure in the recoil subchamber 262. Hydraulic fluid is prevented fromflowing from the recoil subchamber 262 back to the accumulator 290 bythe check valve 334. However, if fluid pressure in the recoil subchamber262 increases beyond the relief setting of the pressure relief valve 336(e.g. 6,000 psi), the relief valve 336 opens thereby allowing fluid tobe advanced from the recoil subchamber 262 to the accumulator 290. Thiscauses rightward movement of the recoil piston 264 (as viewed in FIGS. 9and 10) thereby allowing the slave piston 280 and hence the idler wheel232 to be moved in a rearward direction (i.e. in the general directionof arrow 256 of FIGS. 9-10) thereby providing relief or slack in thedrive track chain 238. It should be appreciated that such relief in thedrive track chain 238 facilitates expulsion of the rock from theundercarriage assembly 226.

Once the rock has been expelled from the undercarriage assembly 226,fluid pressure from the accumulator 290 is returned to the recoilsubchamber 262 thereby again urging the recoil piston leftwardly (asviewed in FIGS. 9 and 10) against the stop 320 which returns the slavepiston 280 and hence the idler wheel 232 to their previous positionsthereby returning the drive track chain 238 to its previous tensionsetting. It should be appreciated that such a configuration of the tracktensioning assembly 248 provides a recoil response with a relativelyefficient hysteresis loop relative to heretofore designed recoilassemblies. In particular, return flow from the recoil subchamber 262 tothe accumulator 290 must flow through the relief valve 336 therebygenerating a relatively large recoil force (e.g. 6,000 psi), however,flow from the accumulator 290 back to the recoil subchamber 262 isallowed to pass unrestricted by the check valve 334 thereby producing arelatively low pressure recoil recovery.

Hence, as described herein, the track tensioning assembly 248 providesnumerous advantages over heretofore designed track tensioningassemblies. For example, by automatically switching between a relativelytaut track configuration and a loosened track configuration, theexcavator 210 is prevented from rolling back and forth during a diggingoperation, but yet also gains the benefit of decreased undercarriagecomponent wear. Moreover, the design of the track tensioning assembly248 facilitates its integration into existing excavator designs. Inparticular, existing excavator designs typically do not include a sourceof pressurized hydraulic fluid within the undercarriage assembly whenthe excavator is performing a digging operation. This is true since theexcavator is generally not advanced during a digging operation therebyeliminating the need for hydraulic pressure within the undercarriageassembly. However, use of the accumulator 290 for the storing ofhydraulic pressure eliminates the need for an active pressure sourcethereby facilitating ease of retrofit of the track tensioning assembly248 into existing excavator designs.

Referring now to FIGS. 11-14, there is shown a second embodiment of atrack tensioning assembly 350 which may be utilized in conjunction withthe excavator 210. As shall be discussed below in greater detail, thetrack tensioning assembly 350 performs similar functions as the tracktensioning assembly 248. In particular, the track tensioning assembly350 is configured to provide a relatively taut track configuration inorder to prevent the excavator 210 from rolling back and forth during adigging operation, but then loosen the tension on the drive track chain238 during advancement of the excavator 210 in order to decreaseundercarriage component wear. Moreover, the track tensioning assembly350 also provides a recoil function thereby eliminating the need toprovide separate recoil components.

The track tensioning assembly 350 includes a control valve 352, apressurized hydraulic fluid source such as a hydraulic pump 354, a slackadjuster device 356, a pair of accumulators 358, 360, and a pressurerelief valve 362. As shown in FIGS. 11 and 12, the track tensioningassembly 350 is fluidly coupled to a pair of actuators such as hydrauliccylinders 364, 366 in order to control position of the front idlerwheels 232 of the excavator 210. As described above, tension on thedrive track chains 238 is adjusted by moving the front idler wheels 232in a forward or backward direction. In particular, when the front idlerwheels 232 are moved in a forward direction (i.e. in the generaldirection of arrow 254 of FIGS. 7-8 and 11-12), tension on the drivetrack chain 238 is increased. Conversely, when the front idler wheels232 are moved in a rearward direction (i.e. in the general direction ofarrow 256 of FIGS. 7-8 and 11-12), tension on the drive track chain 238is decreased.

As shown in FIGS. 11 and 12, a pilot fluid port 368 of the control valve352 is fluidly coupled to the hydraulic drive system 240 of theexcavator 210 via a fluid line 370. As described above in regard to thefluid line 314 of the track tensioning assembly 248, the fluid line 370may be fluidly coupled to any one of numerous components associated withthe hydraulic drive system 240 (or the implement supply circuit) inorder to communicate changes in hydraulic pressure to the pilot fluidport 368 of the control valve 352. As described above, such changes inhydraulic pressure are indicative of the hydraulic drive system 240being switched between its actuated mode of operation in which the drivesystem 240 causes advancement of the excavator 210 and its deactuatedmode of operation in which the drive system 240 does not advance theexcavator 210. In an exemplary embodiment, the fluid line 370 is fluidlycoupled to the fluid inlet line of the parking brake 316. In such aconfiguration, the control valve 352 is placed in an increase-tensionposition (as shown in FIG. 12) by a spring 244 when relatively low fluidpressure is sensed in the fluid inlet line of the parking brake 316.However, if an increase in fluid pressure is sensed in the fluid inletline of the parking brake 316 thereby indicating that the brake 316 isbeing released, the bias of the spring 244 is overcome by fluid pressurein the pilot fluid port 368, and the control valve 352 is positioned inits decrease-tension position, as shown in FIG. 11.

The pump 354 may be embodied as a dedicated pump for providingpressurized hydraulic fluid to the track tensioning assembly 350 only,or alternatively, a pressurized hydraulic fluid source associated withanother system of the excavator 210 may be utilized as the pump 354. Forexample, the pump 354 may be a pump associated with the drive system240. Moreover, as with the track tensioning system 248, the hydraulicpressure source may be embodied as a passive device such as a fluidaccumulator. In an exemplary embodiment, the pump 354 is embodied as thepilot pressure pump for providing pilot fluid pressure to the componentsassociated with the excavator 210.

As shown in FIGS. 13 and 14, the slack adjuster device 356 includes ahousing 372 having a fluid chamber 374, a fluid inlet 376, and a pair offluid outlets 378, 380 defined therein. A pair of pistons 382, 384 arepositioned in the fluid chamber 374. The pistons 382, 384 are eachpositionable between an increase-tension position (as shown in FIG. 13)and a decrease-tension position (as shown in FIG. 14). In particular,the slack adjuster device 356 includes a pair of springs 386 which arecoupled at a first end to the housing 372 and at a second end to thepistons 382, 384. As shown in FIG. 13, the springs 386, along with fluidpressure in the inlet 376, urge the pistons 382, 384 outwardly so as toposition the pistons 382, 384 at opposite end portions of the fluidchamber 374 thereby positioning the pistons 382, 384 in their respectiveincrease-tension positions. However, as shown in FIG. 14, when hydraulicfluid is advanced out of the fluid chamber 374 and the fluid inlet 376,fluid pressure within the fluid outlets 378, 380 overcomes the bias ofthe springs 386 thereby urging the pistons in 382, 384 in an inwarddirection thereby positioning the pistons 382, 384 in their respectivedecrease-tension positions within a central portion of the fluid chamber374.

Movement of the pistons 382, 384 causes actuation of the hydrauliccylinders 364, 366. In particular, each of the pistons 382, 384 has acheck valve 388 positioned within a central passage 416 defined in eachof the pistons 382, 384. As shown in FIG. 14, each of the check valves388 has a closed check position in which a ball 390 is urged intocontact with the valve seat 392 thereby isolating the fluid inlet 376from the fluid outlets 378, 380 (and hence the hydraulic cylinders 364,366). The check valves 388 also have an open check position in which theball 390 is urged off of the valve seat 392 so as to allow fluid toadvance from the fluid inlet 376 to the fluid outlets 378, 380.

The slack adjuster device 356 may be embodied as any slack adjustingdevice which is capable of performing the described functions. Forexample, slack adjusting devices which are particularly useful as theslack adjuster device 356 of the present invention are commerciallyavailable from The BFGoodrich Company of Charlotte, N.C.

As shown in FIGS. 11 and 12, the fluid outlets 378, 380 are fluidlycoupled to the hydraulic cylinders 364, 366, respectively. Inparticular, the fluid outlet 378 is coupled to the hydraulic cylinder364 via a fluid line 394, whereas the fluid outlet 380 is coupled to thehydraulic cylinder 366 via a fluid line 396. The hydraulic cylinders364, 366 each have a cylinder housing 398 having a rod 400 extendingtherefrom. A first end of the rods 400 is secured to a piston (notshown) within the cylinder housings 398, whereas a second end of each ofthe rods 400 is secured to the yokes 252 associated with the front idlerwheels 232. Hence, extension of the rods 400 (i.e. movement of the rods400 in the general direction of arrow 254 relative to the housings 398)causes corresponding movement of the idler wheels 232 thereby increasingtension on the drive track chains 238. Conversely, retraction of therods 400 (i.e. movement of the rods 400 in the general direction ofarrow 256 relative to the housings 398) causes corresponding movement ofthe idler wheels 232 thereby decreasing tension on the drive trackchains 238.

Hence, from the above description, it should be appreciated that whenthe control valve 352 is positioned in its increase-tension position (asshown in FIG. 12) thereby indicating that the excavator 210 is beingmaintained in a stationary position in order to, for example, perform adigging function, pressurized hydraulic fluid is advanced from the pump354 to the fluid chamber 374 of the slack adjuster device 356 via theinlet fluid port 376. Presence of fluid pressure within the fluidchamber 374 along with the bias of the springs 386 urges the pistons382, 384 in opposite outward directions thereby positioning the pistons382, 384 in their respective increase-tension positions at opposite endportions of the fluid chamber 374. Fluid pressure in the fluid chamber374 also causes the check valves 388 to be positioned in theirrespective open check positions thereby allowing pressurized hydraulicfluid to advance from the fluid inlet 376, through the fluid chamber374, and out the fluid outlets 378, 380 to the hydraulic cylinders 364,366. Presence of such hydraulic fluid pressure causes extension of therods 400 (i.e. movement of the rods 400 in the general direction ofarrow 254 relative to the housings 398) thereby causing correspondingmovement of the idler wheels 232 so as to increase tension on the drivetrack chains 238.

As described above in regard to the track tensioning assembly 248, itshould be appreciated that increasing tension on the drive track chain238 during a digging operation facilitates operation of the excavator210. In particular, by removing substantially all of the slack from thedrive track chain 238, the excavator 210 is less likely to roll back andforth within the inner portion of the drive track chain 238. Moreover,the check valves 388 within the slack adjuster device 356 create, inessence, a hydraulic lock which prevents backward flow from thehydraulic cylinders 364, 366 thereby minimizing, if not eliminating,movement of the front idler wheels 232 due to implement forces generatedby the boom assembly 214 and the bucket 212 during operation thereof.This further eliminates undesirable movement of the undercarriageassembly 226 during performance of a work function.

As described above, if an increase in fluid pressure is sensed in thefluid inlet line of the parking brake 316 thereby indicating that thebrake 316 is being released, such an increase in fluid pressure iscommunicated to the pilot fluid port 368 of the control valve 352 viathe fluid line 370 thereby positioning the control valve 352 in itsdecrease-tension position, as shown in FIG. 11. When the control valve352 is positioned in its decrease-tension position, hydraulic fluidwithin the fluid chamber 374 of the slack adjuster device 356 isexhausted or otherwise vented to a reservoir 402. Absence of fluidpressure within the fluid chamber 374 allows the fluid pressure withinthe fluid outlets 378, 380 to retract or otherwise urge the pistons 382,384, respectively, inwardly so as to position the pistons 382, 384 in acentral portion of the fluid chamber 374 thereby positioning the pistons382, 384 in their respective decrease-tension positions. Absence offluid pressure in the fluid chamber 374 also causes the check valves 388to be positioned in their respective closed check positions therebypreventing pressurized hydraulic fluid from advancing from the hydrauliccylinders 364, 366 to the reservoir 402.

Movement of the pistons 382, 384 from their respective increase-tensionpositions to their respective decrease-tension positions causesretraction of the rods 400 (i.e. movement of the rods 400 in the generaldirection of arrow 256 relative to the housings 398) thereby causingcorresponding movement of the idler wheels 232 so as to decrease tensionon the drive track chains 238. The distance in which the idler wheels232 are retracted (i.e. moved in the general direction of arrow 256 ofFIGS. 7-8 and 11-12) corresponds to the stroke length of the pistons382, 384. In an exemplary embodiment, a desirable traveling tracktension level is achieved by retracting the front idler wheels 232approximately ten millimeters from the point at which substantially allof the slack has been removed from the drive track chain 238 (i.e. thepoint at which the front idler wheels 232 are positioned duringperformance of a work function). Hence, the slack adjuster device 356 isconfigured such that each of the pistons 382, 384 has a stroke lengthwhich causes the front idler wheels 232 to be retracted approximatelyten millimeters when the pistons 382, 384 are moved from theirrespective increase-tension positions to their respectivedecrease-tension positions.

As described above, such loosening of the drive track chain 238 prior toadvancement of the excavator 210 provides numerous advantages overheretofore designed excavators. For example, by loosening or otherwisedecreasing tension on the drive track chain 238 by a predeterminedamount prior to advancement of the excavator 210, wear on componentsassociated with the undercarriage assembly 226 is reduced therebyincreasing the efficiency and even the useful life of the excavator 210.

Moreover, it should be appreciated that the design of the tracktensioning system 350 provides a recoil function to the undercarriageassembly 226 thereby eliminating the need for a separate recoil assemblysuch as a spring or the like. In particular, the combination of theaccumulators 358, 360 and the pressure relief valve 362 allows formovement of the front idler wheels 232 in the event that a rock or thelike is ingested by the undercarriage assembly 226 during advancement ofthe excavator 210. In particular, if during advancement of the excavator210, a rock or the like is ingested by the undercarriage assembly 226thereby urging one of the idler wheels 232 rearwardly (i.e. in thegeneral direction of arrow 256 of FIGS. 7-8 and 11-12), hydraulic fluidis advanced out of the corresponding hydraulic cylinder 364, 366 andinto the corresponding accumulator 358, 360. Hydraulic fluid isprevented from flowing through the slack adjuster device 356 and to thereservoir 402 by the check valves 388. Such advancement of hydraulicfluid out of the hydraulic cylinders 364, 366 and into the fluidaccumulators 358, 360, respectively, causes the idler wheels 232 to bemoved in the rearward direction (i.e. in the general direction of arrow256 of FIGS. 7-8 and 11-12) thereby providing relief or slack in thedrive track chain 238. It should be appreciated that such relief in thedrive track chain 238 facilitates expulsion of the rock from theundercarriage assembly 226. Once the rock has been expelled from theundercarriage assembly 226, fluid pressure from the accumulators 358,360 is returned to the corresponding hydraulic cylinder 364, 366,thereby again urging the affected idler wheel 232 in a forward direction(i.e. in the general direction of arrow 254 of FIGS. 7-8 and 11-12) toits previous position thereby returning the drive track chain 238 to itsprevious tension setting.

However, if fluid pressure in the fluid lines 394, 396 increases beyondthe relief setting of the pressure relief valve 362 (e.g. 6,000 psi),the relief valve 362 is urged rightwardly (as viewed in FIGS. 11 and 12)so as to be positioned in an open position thereby allowing fluid to beadvanced to the reservoir 402. This provides additional relief to thecomponents associated with the undercarriage assembly 226 therebypreventing damage thereto. Once pressure in the fluid lines 394, 396decreases below the relief setting of the pressure relief valve (e.g.6,000 psi), the relief valve closes thereby preventing additional fluidfrom being advanced to the reservoir 402. It should be appreciated thatfluid lost during opening of the relief valve 362 may be replaced by theaccumulators 358, 360. Moreover, the slack adjuster device 356 may becycled in order to provide additional fluid, if necessary.

Hence, as described herein, the track tensioning assembly 350 providesnumerous advantages over heretofore designed track tensioningassemblies. For example, by automatically switching between a relativelytaut track configuration and a loosened track configuration, theexcavator 210 is prevented from rolling back and forth during a diggingoperation, but yet also gains the benefit of decreased undercarriagecomponent wear. Moreover, by incorporating a recoil function therein,the track tensioning assembly 350 eliminates the need for a separaterecoil device thereby lowering costs associated with the design of theexcavator 210.

Referring now to FIG. 15, there is shown a third embodiment of a tracktensioning assembly 450 which may be utilized in conjunction with theexcavator 210. As shall be discussed below in greater detail, the tracktensioning assembly 450 performs similar functions as the tracktensioning assemblies 248 and 350. In particular, the track tensioningassembly 450 is configured to provide a relatively taut trackconfiguration in order to prevent the excavator 210 from rolling backand forth during a digging operation, but then loosen the tension on thedrive track chain 238 during advancement of the excavator 210 in orderto decrease undercarriage component wear. Moreover, the track tensioningassembly 450 also provides a recoil function thereby eliminating theneed to provide separate recoil components.

The track tensioning assembly 450 utilizes similar concepts as thosewhich were described above in regard to operation of the tracktensioning control system 158 of the track-type tractor 10. Moreover,the track tensioning assembly 450 utilizes a number of components whichare utilized in the track tensioning assembly 350. The same referencenumerals are utilized to designate components which are common betweenthe track tensioning assemblies 350, 450.

The track tensioning assembly 450 includes a number of position sensors454 which are positioned in order to monitor the position of a number ofundercarriage components relative to one another. In particular, theposition sensors 454 are provided to sense the position of the frontidler wheels 232. As shall be discussed below in greater detail, theposition of the front idler wheels 232 may be utilized to provide“closed loop” control of the tension on the drive track chain 238.

The position sensors 454 may be provided as any type of sensor which iscapable of sensing the position of the front idler wheels 232. Forexample, the position sensors 454 may be embodied as known sensors fordetecting the position of the rods 400 relative to the housings 398. Itshould be appreciated that the position of the idler wheels 232 may beascertained from the position of the rods 400 relative to the housings398. Moreover, in an exemplary embodiment, the position sensors 454 areprovided as linear displacement transducers which senses the lineardistance between a sensing location associated with the front idlerwheel 232 and a sensing location associated with another undercarriagecomponent such as the frame assembly 228. As with the position sensor154, one commercially available sensor which is particularly useful asthe sensor 454 of the present invention is a Series BTL-2 LinearDisplacement Transducer which is commercially available Balluff,Incorporated.

As shown in FIG. 15, the track tensioning assembly 450 also includes anelectrically-actuated control valve assembly 456. The control valveassembly 456 controls actuation of the hydraulic cylinders 364, 366 inorder to increase or decrease tension on the drive track chain 238. Morespecifically, tension on the drive track chains 238 is adjusted bymoving the front idler wheels 232 in a forward or backward direction.For example, when the front idler wheels 232 are moved in a forwarddirection (i.e. in the general direction of arrow 254 of FIGS. 7-8 and15), tension on the drive track chain 238 is increased. Conversely, whenthe front idler wheels 232 are moved in a rearward direction (i.e. inthe general direction of arrow 256 of FIGS. 7-8 and 15), tension on thedrive track chain 238 is decreased. Hence, the control valve assembly456 may be operated to fluidly couple the hydraulic cylinders 364, 366to a pressurized hydraulic fluid source 458 SO as to increase fluidpressure communicated to the hydraulic cylinders 364, 366 therebyincreasing tension on the drive track chain 238, or may alternativelymay be operated to fluidly couple the hydraulic cylinders 364, 366 to areservoir 460 so as to decrease fluid pressure in the hydrauliccylinders 364, 366 thereby decreasing tension on the drive track chain238.

The track tensioning assembly 450 also includes a number of mode sensors462 for sensing the mode of operation in which the excavator 210 isbeing operated. In particular, the mode sensors 462 are provided todetermine if the operator is operating the excavator 210 in either itstravel mode of operation in which the hydraulic excavator 210 is beingadvanced from one location to another or its work mode of operation inwhich the hydraulic excavator 210 is operated to perform a work functionsuch as a digging operation. As described below, the mode sensors 462may be provided as any one of a number of different sensors.

For example, the mode sensor 462 may be provided as a pressure sensorfor sensing a change in hydraulic fluid pressure within the hydraulicdrive circuit of the drive system 240. In such a configuration, the modesensor 462 is utilized to determine that the excavator 210 is beingoperated in its travel mode of operation when fluid pressure within thedrive system 240 is relatively high (e.g. above a predetermined pressurethreshold). Conversely, the mode sensor 462 may also be utilized todetermine that the excavator 210 is being operated in its work mode ofoperation when fluid pressure within the drive system 240 is relativelylow (since the excavator 210 is not advanced during performance of awork function).

The mode sensor 462 may also be provided as a pressure sensor forsensing a change in hydraulic fluid pressure within the implement fluidsupply circuit 464. It should be appreciated that the implement fluidsupply circuit 464 includes the components necessary to supplypressurized hydraulic fluid to the work implements associated with theexcavator 210 such as the boom assembly 214 and the bucket 212. In sucha configuration, the mode sensor 462 is utilized to determine that theexcavator 210 is being operated in its work mode of operation when fluidpressure within the implement fluid supply system 464 is relatively high(e.g. above a predetermined pressure threshold).

The mode sensor 462 may also be embodied as a position sensor forsensing position of one or more of the control devices 242 located inthe cab 240. For example, the mode sensor 462 may be provided as aposition sensor for sensing position of the foot pedal assembly 246which is utilized by the operator to advance the excavator 210. In sucha configuration, the mode sensor 462 is utilized to determine that theexcavator 210 is being operated in its travel mode of operation when thefoot pedal assembly 246 is positioned in a first position such as adepressed pedal position. Conversely, the mode sensor 462 may also beutilized to determine that the excavator 210 is being operated in itswork mode of operation when the foot pedal assembly 246 is positioned ina second position such as a released pedal position (since the excavator210 is not advanced during performance of a work function).

Moreover, the mode sensor 462 may also be embodied as a position sensorfor sensing position of other control devices 242 located in the cab240. For example, the mode sensor 462 may be provided as a positionsensor for sensing position of an implement control lever 466 which isutilized by the operator to operate the implements associated with theexcavator 210 such as the boom assembly 214 and the bucket 212. In sucha configuration, the mode sensor 462 is utilized to determine that theexcavator 210 is being operated in its work mode of operation when thecontrol lever 466 is positioned in a first position such as a forward orbackward lever position. Conversely, the mode sensor 462 may also beutilized to determine that the excavator 210 is being operated in itstravel mode of operation when the control lever 466 is positioned in asecond position such as a neutral lever position (since the implementsassociated with the excavator 210 are not operated during advancement ofthe excavator 210).

It should be appreciated that the above-described embodiments of themode sensor 462 are meant to be exemplary in nature, and that numerousother embodiments of the mode sensor 462 may be utilized to determinewhether the excavator 210 is being operated in its travel mode ofoperation or its work mode of operation. Accordingly, while theparticular embodiments of the mode sensor 462 described herein providesignificant advantages to the present invention, certain of suchadvantages may be realized by utilization of numerous other embodimentsof the mode sensor 462 other than those described herein.

As shown in FIG. 15, the control valve assembly 456, the positionsensors 454, and the mode sensor 462 are each electrically coupled to aprocessing unit such as a controller 470. The controller 470 may be adedicated controller for controlling the components shown in FIG. 15, ormay alternatively be integrated into another controller associated withthe excavator 210 such as the engine controller (not shown),transmission controller (not shown), or implement controller (notshown).

The controller 470 is essentially the same as the controller 160 whichwas described above in regard to the track tensioning control system 158of the track-type tractor 10. In particular, the controller 470 includeselectrical components commonly found in other work machine controllerssuch as a microprocessor 472, a memory device 474, and an interfacecircuit 476. The interface circuit 476 converts the output signals fromthe position sensors 454 and the mode sensor 462 into a signal which issuitable for presentation to an input of the microprocessor 472. Inparticular, an analog-to-digital (A/D) converter (not shown) associatedwith the interface circuit 476 converts the analog voltage or other typeof output signal generated by the position sensors 454 and the modesensor 462 into a digital value for use by the microprocessor 472. Itshould be appreciated that the magnitude of the analog voltage generatedby the position sensor 454 is indicative of the position of the frontidler wheels 232, whereas the magnitude of the analog voltage (or othercharacteristic of the signal) generated by the mode sensor 462 isindicative of the mode of operation of the excavator 210.

The interface circuit 476 also converts output signals generated by themicroprocessor 472 into a signal which is suitable for use by a solenoid478 associated with the control valve assembly 456. In particular, theinterface circuit 476 converts the output signals from themicroprocessor 472 into an analog actuation pulse which actuates thesolenoid 478 thereby positioning the control valve assembly 456 into oneof its actuated positions in order to cause extension or retraction ofthe hydraulic cylinders 364, 366 as described above. It should befurther appreciated that the interface circuit 476 may be embodied as adiscrete device or number of devices, or may be integrated into themicroprocessor 472.

The memory device 474 is provided to store the code or set ofinstructions which are executed by the controller 470 during operationof the track tensioning system 450. Moreover, operation parameters mayalso be stored in the memory device 474. The memory device may beembodied as any known memory device such as RAM and/or ROM devices.

As shown in FIG. 15, the position sensors 454 are electrically coupledto the controller 470 via a pair of signal lines 480. Hence, outputsignals generated by the position sensors 480 are communicated to thecontroller 470 via the signal lines 480. As discussed above, such outputsignals may be generated and thereafter communicated by the positionsensors 454 in numerous forms. For example, the position sensors 454 maygenerate output signals in the form of an analog DC voltage or in theform of a signal utilizing current-to-pulse signal timing.

The control valve assembly 456 is also electrically coupled to thecontroller 470. In particular, the solenoid 478 of the control valveassembly 456 is electrically coupled to the controller 470 via a signalline 482. Hence, the controller 470 generates output signals in the formof actuation pulses on the signal line 482 which actuate the solenoid478 thereby positioning the control valve assembly 456 into a number ofvalve positions which, as described above, selectively extend or retractthe rods 400 relative to the cylinder housings 398 of the hydrauliccylinders 364, 366 thereby selectively increasing or decreasing tensionon the drive track chain 238.

The mode sensor 462 is also electrically coupled to the controller 470via a signal line 484. As described above, the mode sensor 462 isprovided to sense or otherwise determine whether the excavator 210 isbeing operated in either its drive mode of operation in which theexcavator 210 is being advanced from one location to another, or itswork mode of operation in which the excavator 210 is utilized to performa work function such as a digging function. Hence, the mode sensor 462generates output signals on the signal line 484 which are indicative ofthe mode of operation of the excavator 210.

The controller 470 communicates with the position sensors 454, thecontrol valve assembly 456, and the mode sensor 462 in order to maintaina desired level of tension on the drive track chain 238. In particular,the controller 470 initially executes a “zeroing” routine in which thecontroller 470 causes all of the slack to be removed from the drivetrack chain 238. In order to accomplish this, the controller 470generates an output signal on the signal line 482 so as to actuate thesolenoid 478 of the control valve assembly 456 thereby causingpressurized hydraulic fluid to be advanced to the hydraulic cylinders364, 366 from the fluid source 458. The presence of pressurizedhydraulic fluid at the head end of the hydraulic cylinders 364, 366causes the rods 400 of the cylinders 364, 366 to be extended orotherwise moved in the forward direction (i.e. in the general directionof arrow 254 of FIGS. 7-8 and 15). Such forward movement of the rods 400likewise urges the front idler wheels 232 in the forward direction (i.e.in the general direction of arrow 254 of FIGS. 7-8 and 15) therebyincreasing tension on the drive track chain 238. It should beappreciated that the above-described procedure continues untilsubstantially all of the slack has been removed from the drive trackchain 238. It should be appreciated that numerous techniques may beutilized to determine when substantially all of the slack has beenremoved from the drive track chain 238. For example, visual inspectionof the drive track chain 238 may be performed in order to determine whenthe track chain 238 is taut or otherwise has substantially all of theslack removed therefrom. Moreover, a pressure sensor may be utilized todetermine when fluid pressure within the hydraulic cylinders 364, 366increases to a pressure level indicative of substantially all of theslack having been removed from the drive track chain 238. In addition,the position sensors 454 may be utilized to determine if subsequentadditions of hydraulic fluid into the hydraulic cylinders 364, 366 areunable to further extend the rods 400 (i.e. unable to further move thefront idler wheels 232) thereby indicating that substantially all of theslack has been removed from the drive track chain 238.

In any event, once the controller 470 has established a “zero position”(i.e. substantially all of the slack has been removed from the drivetrack chain 238), a predetermined amount of hydraulic fluid is thenremoved from the hydraulic cylinders 364, 366 in order to retract orotherwise move the rods 400 in a rearward direction (i.e. in the generaldirection of arrow 256 of FIGS. 7-8 and 15) by a predetermined distance.In particular, the controller 470 generates an output signal on thesignal line 482 so as to actuate the solenoid 478 of the control valveassembly 456 thereby causing pressurized hydraulic fluid to be drainedfrom the hydraulic cylinders 364, 366 to the reservoir 460. The removalof pressurized hydraulic fluid from the head end of the hydrauliccylinders 364, 366 causes movement of the rods 400 of the cylinders 364,366 in the rearward direction (i.e. in the general direction of arrow256 of FIGS. 7-8 and 15). Such rearward movement of the rods 400likewise urges the front idler wheels 232 in the rearward direction(i.e. in the general direction of arrow 256 of FIGS. 7-8 and 15) therebydecreasing tension on the drive track chain 238. Once the positionsensors 454 detect that the respective front idler wheels 232 have beenmoved rearwardly the predetermined distance, the controller 470 ceasesto generate output signals on the signal line 482 thereby ceasingretraction of the rods 400 of the hydraulic cylinders so as to positionthe front idler wheels 232 in a “traveling position”. It should beappreciated that retraction of the rods 400 of the hydraulic cylinders364, 366 by the predetermined distance creates a correspondingpredetermined amount of slack in the drive track chain 238.

Thereafter, the position sensors 454 are utilized to maintain the frontidler wheels 232 in the traveling position during advancement of theexcavator 210. In particular, if the position sensors 454 detect thatthe position of the one of the front idler wheels 232 changes in amanner which is indicative that tension on the drive track chain 238 hasdecreased, the controller 470 controls operation of the control valveassembly 456 so as to increase fluid pressure in the head end of thecorresponding hydraulic cylinder 364, 366 in the manner described aboveso as to move the affected front idler wheel 232 in the forwarddirection (i.e. in the general direction of arrow 254 of FIGS. 7-8 and15) so as to incrementally increase tension on the drive track chain238. Once the affected front idler wheel 232 has been advanced back toits traveling position (as sensed by the appropriate position sensor454), the controller 470 ceases to increase fluid pressure in the headend of the corresponding hydraulic cylinder 364, 366 thereby ceasingforward advancement of the rod 400 and hence the front idler wheel 232.

Conversely, if the position sensors 454 detect that the position of theone of the front idler wheels 232 changes in a manner which isindicative that tension on the drive track chain has increased, thecontroller 470 controls operation of the control valve assembly 456 soas to decrease fluid pressure in the head end of the correspondinghydraulic cylinder 364, 366 in the manner described above so as to movethe affected front idler wheel 232 in the rearward direction (i.e. inthe general direction of arrow 256 of FIGS. 7-8 and 15) so as toincrementally decrease tension on the drive track chain 238. Once theaffected front idler wheel 232 has been advanced back to its travelingposition (as sensed by the appropriate position sensor 454), thecontroller 470 ceases to decrease fluid pressure in the head end of thecorresponding hydraulic cylinder 364, 366 thereby ceasing rearwardadvancement of the rod 400 and hence the front idler wheel 232

It should be appreciated that such “closed loop” control of the tensionon the drive track chain 238 prevents the excavator 210 from beingadvanced for a period of time with an undesirable amount of tension(either too high or too low) in the drive track chain 238 therebyincreasing the efficiency of the excavator 210 while also increasing theuseful life of the components associated with the undercarriage assembly226. Moreover, it should be appreciated that the controller 470 may alsobe configured to automatically re-execute the “zeroing” procedure atpredetermined intervals so as to account for normal wear in thecomponents associated with the drive track chain 238.

Moreover, it should be appreciated that a pressure relief valve (notshown) may be fluidly interposed between the control valve assembly 456and the hydraulic cylinders 364, 366 in order to provide a recoilfunction to the undercarriage assembly 226. In particular, if a rock orthe like is ingested by the undercarriage assembly 226 duringadvancement of the excavator 210, one of the front idler wheels 232 isurged or otherwise moved rearwardly (i.e. in the general direction ofarrow 256 of FIGS. 7-8 and 15) thereby increasing fluid pressure in thesupply lines from the control valve assembly 456 to the hydrauliccylinders 364, 366. Once fluid pressure within the supply lines isgreater in magnitude than the relief setting of the pressure reliefvalve (e.g. 6,000 psi), hydraulic fluid within the supply lines and theaffected hydraulic cylinder 364, 366 is exhausted to the reservoir 460via the pressure relief valve thereby allowing the rod 400 (and hencethe corresponding front idler wheel 232) to be urged or otherwise movedin the rearward direction (i.e. in the general direction of arrow 256 ofFIGS. 7-8 and 15) thereby providing relief or slack in the affecteddrive track chain 238. It should be appreciated that such relief in thedrive track chain 238 facilitates expulsion of the rock from theundercarriage assembly 226.

Once the rock has been expelled from the undercarriage assembly 226, thefront idler wheel 232 is returned to its previous traveling positionthereby returning the drive track chain 238 to its previous tensionlevel. In particular, the controller 470 controls actuation of thecontrol valve assembly 456 based on output from the position sensors 454in order to return the front idler wheel 232 to its previous travelingposition thereby returning the drive track chain 238 to its previoustension level. It should be appreciated that the controller 470 mayalternatively be configured to execute the “zeroing procedure” so as toreset the traveling position of the front idler wheel 232 after eachrecoil event.

It should also be appreciated that the configuration of the tracktensioning assembly 450 allows for operation of the excavator 210 in asimilar manner as the track tensioning assemblies 248 and 350. Inparticular, the track tensioning assembly 450 is configured to provide arelatively taut track configuration in order to prevent the excavator210 from rolling back and forth during a digging operation, but thenloosen the tension on the drive track chain 238 to a predeterminedtension level during advancement of the excavator 210 in order todecrease undercarriage component wear.

In order to provide such functionality, the controller 470 monitorsoutput from the mode sensor 462. As described above, output from themode sensor 462 is indicative of whether the excavator 210 is beingoperated in its work mode of operation in which the excavator 210 isutilized to perform a work function, or its travel mode of operation inwhich the excavator 210 is advanced from one location to another.

If output from the mode sensor 462 indicates that the excavator 210 isbeing operated in its work mode of operation, the controller 470generates an implement-active control which causes substantially all ofthe slack to be removed from the drive track chain 238. In order toaccomplish this, the controller 470 generates an increase-tensioncontrol signal on the signal line 482 so as to actuate the solenoid 478of the control valve assembly 456 thereby causing pressurized hydraulicfluid to be advanced to the hydraulic cylinders 364, 366 from the fluidsource 458. The presence of pressurized hydraulic fluid at the head endof the hydraulic cylinders 364, 366 causes the rods 400 of the cylinders364, 366 to be extended or otherwise moved in the forward direction(i.e. in the general direction of arrow 254 of FIGS. 7-8 and 15). Suchforward movement of the rods 400 likewise urges the front idler wheels232 in the forward direction (i.e. in the general direction of arrow 254of FIGS. 7-8 and 15) thereby increasing tension on the drive track chain238. It should be appreciated that the above-described procedurecontinues until substantially all of the slack has been removed from thedrive track chain 238. As described above, numerous techniques may beutilized to determine when substantially all of the slack has beenremoved from the drive track chain 238. For example, visual inspectionof the drive track chain 238 may be performed in order to determine whenthe track chain 238 is taut or otherwise has substantially all of theslack removed therefrom. Moreover, a pressure sensor may be utilized todetermine when fluid pressure within the hydraulic cylinders 364, 366increases to a pressure level indicative of substantially all of theslack having been removed from the drive track chain 238. In addition,the position sensors 454 may be utilized to determine if subsequentadditions of hydraulic fluid into the hydraulic cylinders 364, 366 areunable to further extend the rods 400 (i.e. unable to further move thefront idler wheels 232) thereby indicating that substantially all of theslack has been removed from the drive track chain 238.

Once substantially all of the slack has been removed from the drivetrack chain 238, a track-taut control signal is generated and theexcavator 210 may be operated to perform a work function such as adigging operation. In particular, hydraulic pressure from the implementfluid supply circuit 464 is selectively directed to the componentsassociated with the implement assembly of the excavator 210 such as theboom assembly 214 and the bucket 212 in order to perform the workfunction.

Thereafter, if the mode sensor 462 subsequently detects that theexcavator 210 is being operated in its drive mode of operation, thecontroller 470 generates a machine-advancement control signal whichcauses a predetermined amount of slack to be introduced into the drivetrack chain 238. In order to accomplish this, the controller 470 causeshydraulic fluid to be removed from the head end of the hydrauliccylinders 364, 366 in order to retract or otherwise move the rods 400 ina rearward direction (i.e. in the general direction of arrow 256 ofFIGS. 7-8 and 15) by a predetermined distance. In particular, thecontroller 470 generates a decrease-tension control signal on the signalline 482 so as to actuate the solenoid 478 of the control valve assembly456 thereby causing pressurized hydraulic fluid to be drained from thehydraulic cylinders 364, 366 to the reservoir 460. The removal ofpressurized hydraulic fluid from the head end of the hydraulic cylinders364, 366 causes movement of the rods 400 of the cylinders 364, 366 inthe rearward direction (i.e. in the general direction of arrow 256 ofFIGS. 7-8 and 15). Such rearward movement of the rods 400 likewise urgesthe front idler wheels 232 in the rearward direction (i.e. in thegeneral direction of arrow 256 of FIGS. 7-8 and 15) thereby decreasingtension on the drive track chain 238. Once the position sensors 454detect that the respective front idler wheels 232 have been movedrearwardly by the predetermined distance, the controller 470 ceases togenerate output signals on the signal line 482 thereby ceasingretraction of the rods 400 of the hydraulic cylinders 364, 366 so as toposition the front idler wheels 232 in their respective travelingpositions. It should be appreciated that retraction of the rods 400 ofthe hydraulic cylinders 364, 366 by the predetermined distance creates acorresponding predetermined amount of slack in the drive track chain238. Once the front idler wheels 232 have been positioned in theirrespective traveling positions, a tension-reduced control signal isgenerated and the hydraulic components associated with the drive system240 may be utilized to advance the excavator 210 in the desireddirection. During such advancement of the excavator 210, the controller470 monitors output from the position sensors 454 in order to maintainthe front idler wheels 232 in their respective traveling positions inthe manner previously discussed.

Hence, as described herein, the track tensioning assembly 450 providesnumerous advantages over heretofore designed track tensioningassemblies. For example, by automatically switching between a relativelytaut track configuration and a loosened track configuration, theexcavator 210 is prevented from rolling back and forth during a diggingoperation, but yet also gains the benefit of decreased undercarriagecomponent wear during advancement of the excavator 210. Moreover, byincorporating a recoil function therein, the track tensioning assembly450 eliminates the need for a separate recoil device thereby loweringcosts associated with the design of the excavator 210.

Industrial Applicability

In particular regard to operation of the track-type tractor 10, thecontroller 160 of the track tensioning control system 158 communicateswith the position sensor 154, the master valve assembly 90, the inletcontrol valve 142, and the outlet control valve 144 in order to maintaina desired level of tension on the drive track chain 28. In particular,the controller 160 initially executes a “zeroing” routine in which thecontroller 160 causes substantially all of the slack to be removed fromthe drive track chain 28 in the manner described above. Once thecontroller 160 has established a “zero position” (i.e. substantially allof the slack has been removed from the drive track chain 28), apredetermined amount of hydraulic fluid is then removed from the fluidchamber 54 in order to retract or otherwise move the piston 48 in arearward direction (i.e. in the general direction of arrow 44 of FIGS. 2and 4) by a predetermined distance so as to position the front framemember 32 (and hence the front idler wheel 22) in a “target position”.It should be appreciated that retraction of the piston 48 by thepredetermined distance creates a corresponding predetermined amount ofslack in the drive track chain 28.

Thereafter, the position sensor 154 is utilized to maintain the frontframe member 32 (and hence the front idler wheel 22) in the targetposition. In particular, if the position sensor 154 detects that thelinear distance D from the front frame member 32 to the rear framemember 34 decreases thereby indicating that tension on the drive trackchain has decreased, the controller 160 controls operation of the mastervalve assembly 90 and the inlet control valve 142 so as to increasefluid pressure in the fluid chamber 54 of the actuator 48 in the mannerdescribed above so as to move the front frame member 32 in the forwarddirection (i.e. in the general direction of arrow 42 of FIGS. 2 and 4)so as to incrementally increase tension on the drive track chain 28.Once the front frame member 32 has been advanced back to the targetposition (as sensed by the position sensor 154), the controller 160ceases to increase fluid pressure in the fluid chamber 54 therebyceasing forward advancement of the piston 48.

Conversely, if the position sensor 154 detects that the linear distanceD from the front frame member 32 to the rear frame member 34 increasesthereby indicating that tension on the drive track chain has increased,the controller 160 controls operation of the master valve assembly 90and the outlet control valve 144 so as to incrementally decrease fluidpressure in the fluid chamber 54 of the actuator 48 in the mannerdescribed above so as to move the front frame member 32 in the rearwarddirection (i.e. in the general direction of arrow 44 of FIGS. 2 and 4)so as to decrease tension on the drive track chain 28. Once the frontframe member 32 has been advanced back to the target position (as sensedby the position sensor 154), the controller 160 ceases to decrease fluidpressure in the fluid chamber 54 thereby ceasing rearward advancement ofthe piston 48.

As described in detail above, such “closed loop” control of the tensionon the drive track chain 28 prevents the track-type tractor 10 frombeing operated for a period of time with an undesirable amount oftension (either too high or too low) in the drive track chain 28 therebyincreasing the efficiency of the track-type tractor 10 while alsoincreasing the useful life of the components associated with theundercarriage assembly 16. Moreover, it should be appreciated that thecontroller 160 may also be configured to automatically re-execute the“zeroing” procedure at predetermined intervals so as to account fornormal wear in the components associated with the drive track chain 28.

Moreover, as described in detail above, the use of a pressure reliefvalve (not shown) which is fluidly interposed between the fluid port 138of the valve group 58 and the fluid port 140 of the actuator 46 providesa recoil function to the undercarriage assembly 16. In particular, if arock or the like is ingested by the undercarriage assembly 16 duringadvancement of the track-type tractor 10, the front idler wheel 22 isurged or otherwise moved rearwardly (i.e. in the general direction ofarrow 44 of FIGS. 2 and 4) thereby increasing fluid pressure in thefluid chamber 54 of the actuator 46. Once fluid pressure within thefluid chamber 54 is greater in magnitude than the relief setting of thepressure relief valve (e.g. 6,000 psi), hydraulic fluid within the fluidchamber 54 is exhausted to the reservoir 72 via the pressure reliefvalve thereby allowing the piston 48 (and hence the front frame member32 and the front idler wheel 22) to be urged or otherwise moved in therearward direction (i.e. in the general direction of arrow 44 of FIGS. 2and 4) thereby providing relief or slack in the drive track chain 28. Itshould be appreciated that such relief in the drive track chain 28facilitates expulsion of the rock from the undercarriage assembly 16.

Once the rock has been expelled from the undercarriage assembly 16, thefront frame member 32 is returned to its previous target positionthereby returning the drive track chain 28 to its previous tensionlevel. In particular, the controller 160 controls actuation of thevalves 90, 142, and 144 based on output from the position sensor 154 inorder to return the front frame member 32 (and hence the front idlerwheel 22) to its previous target position thereby returning the drivetrack chain 28 to its previous tension level.

In regard to operation of the hydraulic excavator 210 equipped with thetrack tensioning assembly 248, actuation or deactuation of the hydraulicdrive system 240 may be monitored in order to determine the mode ofoperation in which the excavator 210 is being operated. In particular,fluid pressure communicated from the hydraulic drive system 240 may beutilized to determine if the excavator 210 is being operated in a workmode of operation in which the excavator 210 is utilized to perform awork function such as a digging function, or a travel mode of operationin which the excavator 210 is advanced from one location to another.When the excavator 210 is being operated in its work mode of operation,the control valve assembly 288 is positioned in its increase-tensionposition, as shown in FIG. 10. When the control valve assembly 288 ispositioned in its increase-tension position, the master piston 276 isurged leftwardly (as viewed in FIGS. 9 and 10) in the manner describedabove so as to increase fluid pressure in the main chamber 260. Such anincrease in fluid pressure in the main chamber 260 urges the head endportion 282 of the slave piston 280 in the forward direction (i.e. inthe general direction of arrow 254 of FIGS. 8-10) thereby likewisemoving the front idler wheel 232 in the forward direction untilsubstantially all of the slack is removed from the drive track chain 238(i.e. the track is taut). It should be appreciated that such an increasein tension on the drive track chain 238 during a digging operationfacilitates operation of the excavator 210 by rendering the excavator210 less likely to roll back and forth within the inner portion of thedrive track chain 238.

If an increase in fluid pressure is sensed in the fluid line 314,thereby indicating that the excavator 210 is being operated in itstravel mode of operation, the control valve assembly 288 is positionedin its decrease-tension position, as shown in FIG. 9. When the controlvalve assembly 288 is positioned in its decrease-tension position,hydraulic fluid within the master chamber 272 is exhausted or otherwisevented to the reservoir 308 thereby decreasing fluid pressure in themain chamber 260. Such a decrease in fluid pressure in the main chamber260 causes movement of the master piston 276 in a rightwardly direction(as viewed in FIGS. 9 and 10) until movement of the master piston 276 isstopped by a stop 328. Such rightward movement of the master piston 276decreases fluid pressure in the main chamber 260 so as to urge the headend portion 282 of the slave piston 280 in the rearward direction (i.e.in the general direction of arrow 256 of FIGS. 8-10) thereby likewisemoving the front idler wheel 232 in the rearward direction. The distancein which the idler wheel 232 is retracted (i.e. moved in the generaldirection of arrow 256 of FIGS. 8-10) corresponds to the stroke lengthof the master piston 276. As described above, hydraulic fluid lostduring venting of the master chamber 272 is replaced in the accumulatorby use of hydraulic fluid from the drive system 240.

As described above, the design of the track tensioning system 248provides a recoil function to the undercarriage assembly 226 therebyeliminating the need for a separate recoil assembly such as a spring orthe like. In particular, the combination of the check valve 334 and thepressure relief valve 336 allows for selective movement of the recoilpiston 264 in the event that a rock or the like is ingested by theundercarriage assembly 226 during advancement of the excavator 210. Suchmovement of the recoil piston 264 causes corresponding movement of thefront idler wheel 232. Hence, if during advancement of the excavator210, a rock or the like is ingested by the undercarriage assembly 226thereby urging the idler wheel 232 and hence the piston head end 282 ofthe slave piston rearwardly (i.e. in the general direction of arrow 256of FIGS. 8-10), fluid pressure is exerted on a first end 338 of therecoil piston 264 thereby increasing fluid pressure in the recoilsubchamber 262. Hydraulic fluid is prevented from flowing from therecoil subchamber 262 back to the accumulator 290 by the check valve 334(see FIG. 9). However, if fluid pressure in the recoil subchamber 262increases beyond the relief setting of the pressure relief valve 336(e.g. 6,000 psi), the relief valve 336 opens thereby allowing fluid tobe advanced from the recoil subchamber 262 to the accumulator 290. Thiscauses rightward movement of the recoil piston 264 (as viewed in FIGS. 9and 10) thereby allowing the slave piston 280 and hence the idler wheel232 to be moved in a rearward direction (i.e. in the general directionof arrow 256 of FIGS. 9-10) thereby providing relief or slack in thedrive track chain 238. It should be appreciated that such relief in thedrive track chain 238 facilitates expulsion of the rock from theundercarriage assembly 226. Once the rock has been expelled from theundercarriage assembly 226, fluid pressure from the accumulator 290 isreturned to the recoil subchamber 262 thereby again urging the recoilpiston leftwardly (as viewed in FIGS. 9 and 10) against the stop 320which returns the slave piston 280 and hence the idler wheel 232 to itsprevious position thereby returning the drive track chain 238 to itsprevious tension setting.

In regard to operation of the hydraulic excavator 210 equipped with thetrack tensioning assembly 350, actuation or deactuation of the hydraulicdrive system 240 may be monitored in order to determine the mode ofoperation in which the excavator 210 is being operated in a similarmanner as to that described above in regard to the track tensioningassembly 248. In particular, fluid pressure communicated from thehydraulic drive system 240 may be utilized to determine if the excavator210 is being operated in a work mode of operation in which the excavator210 is utilized to perform a work function such as a digging function,or a travel mode of operation in which the excavator 210 is advancedfrom one location to another. When the excavator 210 is being operatedin its work mode of operation, the control valve 352 is positioned inits increase-tension position, as shown in FIG. 12. When the controlvalve 352 is positioned in its increase-tension position, pressurizedhydraulic fluid is advanced from the pump 354 to the fluid chamber 374of the slack adjuster device 356. Presence of fluid pressure within thefluid chamber 374, along with the bias of the springs 386, urges thepistons 382, 384 in opposite outward directions so as to position thepistons 382, 384 in their respective increase-tension positions atopposite end portions of the fluid chamber 374. Fluid pressure in thefluid chamber 374 also causes the check valves 388 to be positioned intheir respective open check positions thereby allowing pressurizedhydraulic fluid to advance to the hydraulic cylinders 364, 366 so as tocause extension of the rods 400 (i.e. movement of the rods 400 in thegeneral direction of arrow 254 relative to the housings 398) therebycausing corresponding movement of the idler wheels 232 so as to increasetension on the drive track chains 238.

If an increase in fluid pressure is sensed in the fluid line 370 therebyindicating that the excavator 210 is being operated in its travel modeof operation, the control valve 352 is positioned in itsdecrease-tension position, as shown in FIG. 11. When the control valve352 is positioned in its decrease-tension position, hydraulic fluidwithin the fluid chamber 374 of the slack adjuster device 356 isexhausted or otherwise vented to the reservoir 402 thereby causingretraction of the rods 400 (i.e. movement of the rods 400 in the generaldirection of arrow 256 relative to the housings 398) which causescorresponding movement of the idler wheels 232 so as to decrease tensionon the drive track chains 238. The distance in which the idler wheels232 are retracted (i.e. moved in the general direction of arrow 256 ofFIGS. 7-8 and 11-12) corresponds to the stroke length of the pistons382, 384 thereby achieving a desirable traveling track tension level.

The design of the track tensioning system 350 provides a recoil functionto the undercarriage assembly 226 thereby eliminating the need for aseparate recoil assembly such as a spring or the like. In particular,the combination of the accumulators 358, 360 and the pressure reliefvalve 362 allows for movement of the front idler wheels 232 in the eventthat a rock or the like is ingested by the undercarriage assembly 226during advancement of the excavator 210. In particular, if duringadvancement of the excavator 210, a rock or the like is ingested by theundercarriage assembly 226 thereby urging the idler wheel 232 rearwardly(i.e. in the general direction of arrow 256 of FIGS. 7-8 and 11-12),hydraulic fluid is advanced out of the hydraulic cylinders 364, 366 andinto the accumulators 358, 360, respectively. Hydraulic fluid isprevented from flowing through the slack adjuster device 356 and to thereservoir 402 by the check valves 388. Such advancement of hydraulicfluid out of the hydraulic cylinders 364, 366 and into the fluidaccumulators 358, 360, respectively, causes the idler wheels 232 to bemoved in the rearward direction (i.e. in the general direction of arrow256 of FIGS. 7-8 and 11-12) thereby providing relief or slack in thedrive track chain 238. It should be appreciated that such relief in thedrive track chain 238 facilitates expulsion of the rock from theundercarriage assembly 226. Once the rock has been expelled from theundercarriage assembly 226, fluid pressure from the accumulators 358,360 is returned to the hydraulic cylinders 364, 366, respectively,thereby again urging the idler wheels 232 in a forward direction (i.e.in the general direction of arrow 254 of FIGS. 7-8 and 11-12) to theirprevious position thereby returning the drive track chain 238 to itsprevious tension setting.

However, if fluid pressure in the fluid lines 394, 396 increases beyondthe relief setting of the pressure relief valve 362 (e.g. 6,000 psi),the relief valve 362 opens thereby allowing fluid to be advanced to thereservoir 402. This provides additional relief to the componentsassociated with the undercarriage assembly 226 thereby preventing damagethereto. Once pressure in the fluid lines 394, 396 decreases below therelief setting of the pressure relief valve (e.g. 6,000 psi), the reliefvalve closes thereby preventing additional fluid from being advanced tothe reservoir 402. It should be appreciated that fluid lost duringopening of the relief valve 362 may be replaced by the accumulators 358,360. Moreover, the slack adjuster device 356 may be cycled in order toprovide additional fluid, if necessary.

In regard to operation of the hydraulic excavator 210 equipped with thetrack tensioning assembly 450, the controller 470 monitors output fromthe mode sensor 462 in order to determine whether the excavator 210 isbeing operated in its work mode of operation in which the excavator 210is utilized to perform a work function, or its travel mode of operationin which the excavator 210 is advanced from one location to another. Ifoutput from the mode sensor 462 indicates that the excavator 210 isbeing operated in its work mode of operation, the controller 470 causessubstantially all of the slack to be removed from the drive track chain238 by generating an output signal on the signal line 482 which actuatesthe solenoid 478 of the control valve assembly 456 thereby causingpressurized hydraulic fluid to be advanced to the hydraulic cylinders364, 366 from the fluid source 458. The presence of pressurizedhydraulic fluid at the head end of the hydraulic cylinders 364, 366causes the rods 400 of the cylinders 364, 366 to be extended orotherwise moved in the forward direction (i.e. in the general directionof arrow 254 of FIGS. 7-8 and 15). Such forward movement of the rods 400likewise urges the front idler wheels 232 in the forward direction (i.e.in the general direction of arrow 254 of FIGS. 7-8 and 15) therebyincreasing tension on the drive track chain 238 until substantially allof the slack has been removed from the drive track chain 238.

Once substantially all of the slack has been removed from the drivetrack chain 238, the excavator 210 may be operated to perform a workfunction such as a digging operation. In particular, hydraulic pressurefrom the implement fluid supply circuit 464 is selectively directed tothe components associated with the implement assembly of the excavator210 such as the boom assembly 214 and the bucket 212 in order to performthe work function.

Thereafter, if the mode sensor 462 subsequently detects that theexcavator 210 is being operated in its drive mode of operation, thecontroller 470 causes a predetermined amount of slack to be introducedinto the drive track chain 238 by causing hydraulic fluid to be removedfrom the hydraulic cylinders 364, 366 in order to retract or otherwisemove the rods 400 in a rearward direction (i.e. in the general directionof arrow 256 of FIGS. 7-8 and 15) by a predetermined distance. Inparticular, the controller 470 communicates with the control valveassembly 456 so as to cause pressurized hydraulic fluid to be drainedfrom the hydraulic cylinders 364, 366 to the reservoir 460. The removalof pressurized hydraulic fluid from the head end of the hydrauliccylinders 364, 366 causes movement of the rods 400 of the cylinders 364,366 in the rearward direction (i.e. in the general direction of arrow256 of FIGS. 7-8 and 15). Such rearward movement of the rods 400likewise urges the front idler wheels 232 in the rearward direction(i.e. in the general direction of arrow 256 of FIGS. 7-8 and 15) therebydecreasing tension on the drive track chain 238. Once the positionsensors 454 detect that the front idler wheels 232 have been movedrearwardly by the predetermined distance, the controller 470 ceases togenerate output signals on the signal line 482 thereby ceasingretraction of the rods 400 of the hydraulic cylinders 364, 366 so as toposition the front idler wheels 232 in their respective travelingpositions. It should be appreciated that retraction of the rods 400 ofthe hydraulic cylinders 364, 366 by the predetermined distance creates acorresponding predetermined amount of slack in the drive track chain238. Once the front idler wheels 232 have been positioned in theirrespective traveling positions, the hydraulic components associated withthe drive system 240 may be utilized to advance the excavator 210 in thedesired direction. During such advancement of the excavator 210, thecontroller 470 monitors output from the position sensors 454 in order tomaintain the front idler wheels 232 in their respective travelingpositions in the manner previously discussed.

Moreover, during advancement of the excavator 210, the track tensioningassembly 450 provides a recoil function to the undercarriage assembly226. In particular, if a rock or the like is ingested by theundercarriage assembly 226 during advancement of the excavator 210, oneof the front idler wheels 232 is urged or otherwise moved rearwardly(i.e. in the general direction of arrow 256 of FIGS. 7-8 and 15) therebyincreasing fluid pressure in the supply lines from the control valveassembly 456 to the hydraulic cylinders 364, 366. Once fluid pressurewithin the supply lines is greater in magnitude (e.g. 6,000 psi) thanthe relief setting of the pressure relief valve (not shown) which isinterposed between the control valve assembly 456 and the hydrauliccylinders 364, hydraulic fluid within the supply lines and the affectedhydraulic cylinder 364, 366 is exhausted to the reservoir 460 via thepressure relief valve thereby allowing the rod 400 (and hence thecorresponding front idler wheel 232) to be urged or otherwise moved inthe rearward direction (i.e. in the general direction of arrow 256 ofFIGS. 7-8 and 15) thereby providing relief or slack in the drive trackchain 238. It should be appreciated that such relief in the drive trackchain 238 facilitates expulsion of the rock from the undercarriageassembly 16. Once the rock has been expelled from the undercarriageassembly 226, the front idler wheel 232 is returned to its previoustraveling position thereby returning the drive track chain 238 to itsprevious tension level. In particular, the controller 470 controlsactuation of the control valve assembly 456 based on output from theposition sensors 454 in order to return the affected front idler wheel232 to its previous traveling position thereby returning the drive trackchain 238 to its previous tension level.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that only the preferred embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the invention are desired to be protected.

There are a plurality of advantages of the present invention arisingfrom the various features of the work machines described herein. It willbe noted that alternative embodiments of the work machines of thepresent invention may not include all of the features described yetstill benefit from at least some of the advantages of such features.Those of ordinary skill in the art may readily devise their ownimplementations of the work machines that incorporate one or more of thefeatures of the present invention and fall within the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A valve assembly for controlling actuation of anactuator of a track tensioning system for adjusting tension on a drivetrack chain, comprising: a valve housing defining (i) a master chamber,(ii) a slave chamber, (iii) a fluid passage which fluidly couples saidmaster chamber to said slave chamber, and (iv) a fluid outlet whichfluidly couples said slave chamber to said actuator; a master valvemember positioned within said master chamber, said master valve member(i) isolates said master chamber from said slave chamber when saidmaster valve member is positioned in a first master position, and (ii)places said master chamber in fluid communication with said slavechamber when said master valve member is positioned in a second masterposition; and a slave piston positioned within said slave chamber, saidslave piston (i) isolates said fluid outlet from said slave chamber whensaid slave piston is positioned in a first slave position, and (ii)places said fluid outlet in fluid communication with said slave chamberwhen said slave piston is positioned in a second slave position, whereinmovement of said master valve member from said first master position tosaid second master position causes movement of said slave piston fromsaid first slave position to said second slave position, and whereinmovement of said slave piston from said first slave position to saidsecond slave position causes actuation of said actuator, whereby tensionon said drive track chain is adjusted by said actuator.
 2. The valveassembly of claim 1, further comprising a biasing spring which biasessaid slave piston into said first slave position.
 3. The valve assemblyof claim 1, further comprising an outlet check valve, wherein: saidoutlet check valve has an open check position and a closed checkposition, said slave chamber is fluidly coupled to said fluid outletwhen said outlet check valve is positioned in said open check position,said slave chamber is isolated from said fluid outlet when said outletcheck valve is positioned in said closed check position, and movement ofsaid slave piston from said first slave position to said second slaveposition causes movement of said outlet check valve from said closedcheck position to said open check position.
 4. The valve assembly ofclaim 3, wherein said outlet check valve includes a biasing spring whichbiases said outlet check valve into said closed check position.
 5. Thevalve assembly of claim 1, wherein: said slave piston has (i) a firstfluid contact surface having a first diameter, and (ii) a second fluidcontact surface having a second diameter, hydraulic fluid advanced fromsaid master chamber contacts said first fluid contact surface of saidslave piston, hydraulic fluid advanced from said slave chamber to saidfluid outlet contacts said second fluid contact surface of said slavepiston, and said first diameter is greater than said second diameter. 6.The valve assembly of claim 1, wherein: said actuator includes a fluidcylinder having a cylinder housing with a rod extending therefrom, saidrod being mechanically coupled to an idler wheel, said drive track chainis advanced around said idler wheel, and said rod is moved relative tosaid cylinder housing so as to move said idler wheel thereby adjustingtension on said drive track chain when said slave piston is moved fromsaid first slave position to said second slave position.
 7. Anundercarriage assembly of a work machine, comprising: a pressurizedhydraulic fluid source; a drive track chain; an actuator mechanicallycoupled to said drive track chain so as to adjust tension on said drivetrack chain; and a valve assembly for controlling actuation of saidactuator, said valve assembly having: (i) a valve housing defining (a) afluid chamber, (b) a chamber inlet, and (c) a chamber outlet, (ii) apiston positioned within said fluid chamber so as to divide said fluidchamber into a spring chamber portion and a high-pressure chamberportion, and (iii) a biasing spring positioned in said spring chamberportion so as to exert a spring bias on an outer surface of said piston,wherein (i) said chamber inlet fluidly couples said hydraulic fluidsource to said spring chamber portion, (ii) said chamber outlet fluidlycouples said high-pressure chamber portion to said actuator, (iii) saidpiston has a first fluid contact surface which is positioned in saidspring chamber portion, (iv) said piston further has a second fluidcontact surface which is positioned in said high-pressure chamberportion, (v) said first fluid contact surface has a first diameter, (vi)said second fluid contact surface has a second diameter, and (vii) saidfirst diameter is greater than said second diameter.
 8. Theundercarriage assembly of claim 7, wherein: a first fluid pressure isexerted on said first fluid contact surface of said piston bypressurized hydraulic fluid within said spring chamber portion so as togenerate a second fluid pressure in said high-pressure chamber portion,and said second fluid pressure is greater than said first fluidpressure.
 9. The undercarriage assembly of claim 8, further comprisingan outlet check valve, wherein: said outlet check valve is position insaid chamber outlet, said outlet check valve has an open check positionand a closed check position, said fluid chamber is fluidly coupled tosaid actuator when said outlet check valve is positioned in said opencheck position, said fluid chamber is isolated from said actuator whensaid outlet check valve is positioned in said closed check position,presence of said second fluid pressure in said high-pressure chamberportion positions said outlet check valve in said open check position,and absence of said second fluid pressure in said high-pressure chamberportion positions said outlet check valve in said closed check position.10. The undercarriage assembly of claim 9, further comprising a controlvalve fluidly coupled to said high-pressure chamber portion, wherein:said control valve has an open control position and a closed controlposition, said actuator increases tension on said track chain when (i)said outlet check valve is positioned in said open check position, and(ii) said control valve is positioned in said closed control position,and said actuator decreases tension on said track chain when (i) saidoutlet check valve is positioned in said open check position, and (ii)said control valve is positioned in said open control position.
 11. Theundercarriage assembly of claim 10, further comprising an idler wheel,wherein: said actuator includes a fluid cylinder having a cylinderhousing with a rod extending therefrom, said rod being mechanicallycoupled to said idler wheel, said drive track chain is advanced aroundsaid idler wheel, said rod is moved relative to said cylinder housing ina first direction so as to urge said idler wheel in said first directionthereby increasing tension on said drive track chain when (i) saidoutlet check valve is positioned in said open check position, and (ii)said control valve is positioned in said closed control position, andsaid rod is moved relative to said cylinder housing in a seconddirection so as to urge said idler wheel in said second directionthereby decreasing tension on said drive track chain when (i) saidoutlet check valve is positioned in said open check position, and (ii)said control valve is positioned in said open control position.
 12. Theundercarriage assembly of claim 11, further comprising (i) a controller,and (ii) a position sensor for sensing position of said idler wheel,wherein: both said control valve and said position sensor areelectrically coupled to said controller, said position sensor generatesoutput signals indicative of said position of said idler wheel, and saidcontroller moves said control valve between said open control positionand said closed control position based on said output signals generatedby said position sensor.
 13. An undercarriage assembly of a workmachine, comprising: a first frame member; a second frame member whichis movable relative to said first frame member; an actuator mechanicallycoupled to said second frame member so as to move said second framemember relative to said first frame member; an idler wheel rotatablysecured to said second frame member; a drive track chain which isadvanced around said idler wheel; a pressurized hydraulic fluid source;and a valve assembly for controlling actuation of said actuator, saidvalve assembly having: (i) a valve housing defining (a) a fluid chamber,(b) a chamber inlet, and (c) a chamber outlet, (ii) a piston positionedwithin said fluid chamber so as to divide said fluid chamber into aspring chamber portion and a high-pressure chamber portion, and (iii) abiasing spring positioned in said spring chamber portion so as to exerta spring bias on an outer surface of said piston, wherein (i) saidchamber inlet fluidly couples said hydraulic fluid source to said springchamber portion, (ii) said chamber outlet fluidly couples saidhigh-pressure chamber portion to said actuator, (iii) said piston has afirst fluid contact surface which is positioned in said spring chamberportion, (iv) said piston further has a second fluid contact surfacewhich is positioned in said high-pressure chamber portion, (v) saidfirst fluid contact surface has a first diameter, (vi) said second fluidcontact surface has a second diameter, and (vii) said first diameter isgreater than said second diameter.
 14. The undercarriage assembly ofclaim 13, wherein: a first fluid pressure is exerted on said first fluidcontact surface of said piston by pressurized hydraulic fluid withinsaid spring chamber portion so as to generate a second fluid pressure insaid high-pressure chamber portion, and said second fluid pressure isgreater than said first fluid pressure.
 15. The undercarriage assemblyof claim 14, further comprising an outlet check valve, wherein: saidoutlet check valve is position in said chamber outlet, said outlet checkvalve has an open check position and a closed check position, said fluidchamber is fluidly coupled to said actuator when said outlet check valveis positioned in said open check position, said fluid chamber isisolated from said actuator when said outlet check valve is positionedin said closed check position, presence of said second fluid pressure insaid high-pressure chamber portion positions said outlet check valve insaid open check position, and absence of said second fluid pressure insaid high-pressure chamber portion positions said outlet check valve insaid closed check position.
 16. The undercarriage assembly of claim 15,further comprising a control valve fluidly coupled to said high-pressurechamber portion, wherein: said control valve has an open controlposition and a closed control position, said actuator moves said secondframe member in a direction away from said first frame member so as toincrease tension on said track chain when (i) said outlet check valve ispositioned in said open check position, and (ii) said control valve ispositioned in said closed control position, and said actuator moves saidsecond frame member in a direction toward said first frame member so asto decrease tension on said track chain when (i) said outlet check valveis positioned in said open check position, and (ii) said control valveis positioned in said open control position.
 17. The undercarriageassembly of claim 16, wherein: said actuator includes a fluid cylinderhaving a cylinder housing with a rod extending therefrom with (i) saidcylinder housing being secured to said first frame member, and (ii) saidrod being secured to said second frame member, said rod is movedrelative to said cylinder housing in said direction away from said firstframe member so as to urge said second frame member in said directionaway from said first frame member thereby increasing tension on saiddrive track chain when (i) said outlet check valve is positioned in saidopen check position, and (ii) said control valve is positioned in saidclosed control position, and said rod is moved relative to said cylinderhousing in said direction toward said first frame member so as to urgesaid second frame member in said direction toward said first framemember thereby decreasing tension on said drive track chain when (i)said outlet check valve is positioned in said open check position, and(ii) said control valve is positioned in said open control position. 18.The undercarriage assembly of claim 17, further comprising (i) acontroller, and (ii) a position sensor for sensing position of saidsecond frame member relative to said first frame member, wherein: bothsaid control valve and said position sensor are electrically coupled tosaid controller, said position sensor generates output signalsindicative of said position of said second frame member relative to saidfirst frame member, and said controller moves said control valve betweensaid open control position and said closed control position based onsaid output signals generated by said position sensor.